diff --git a/README.md b/README.md
index 76b7369..6df5ee5 100644
--- a/README.md
+++ b/README.md
@@ -105,6 +105,38 @@ In [3]: pyopenjtalk.g2p("こんにちは", kana=True)
 Out[3]: 'コンニチワ'
 ```
 
+### Create/Apply user dictionary
+
+1. Create a CSV file (e.g. `user.csv`) and write custom words like below:
+
+```csv
+ＧＮＵ,,,1,名詞,一般,*,*,*,*,ＧＮＵ,グヌー,グヌー,2/3,*
+```
+
+2. Call `mecab_dict_index` to compile the CSV file.
+
+```python
+In [1]: import pyopenjtalk
+
+In [2]: pyopenjtalk.mecab_dict_index("user.csv", "user.dic")
+reading user.csv ... 1
+emitting double-array: 100% |###########################################|
+
+done!
+```
+
+3. Call `update_global_jtalk_with_user_dict` to apply the user dictionary.
+
+```python
+In [3]: pyopenjtalk.g2p("GNU")
+Out[3]: 'j i i e n u y u u'
+
+In [4]: pyopenjtalk.update_global_jtalk_with_user_dict("user.dic")
+
+In [5]: pyopenjtalk.g2p("GNU")
+Out[5]: 'g u n u u'
+```
+
 ### About `run_marine` option
 
 After v0.3.0, the `run_marine` option has been available for estimating the Japanese accent with the DNN-based method (see [marine](https://github.com/6gsn/marine)). If you want to use the feature, please install pyopenjtalk as below;
diff --git a/pyopenjtalk/__init__.py b/pyopenjtalk/__init__.py
index 299ac29..72f5db5 100644
--- a/pyopenjtalk/__init__.py
+++ b/pyopenjtalk/__init__.py
@@ -19,6 +19,7 @@
 
 from .htsengine import HTSEngine
 from .openjtalk import OpenJTalk
+from .openjtalk import mecab_dict_index as _mecab_dict_index
 from .utils import merge_njd_marine_features
 
 # Dictionary directory
@@ -224,3 +225,44 @@ def make_label(njd_features):
         _lazy_init()
         _global_jtalk = OpenJTalk(dn_mecab=OPEN_JTALK_DICT_DIR)
     return _global_jtalk.make_label(njd_features)
+
+
+def mecab_dict_index(path, out_path, dn_mecab=None):
+    """Create user dictionary
+
+    Args:
+        path (str): path to user csv
+        out_path (str): path to output dictionary
+        dn_mecab (optional. str): path to mecab dictionary
+    """
+    global _global_jtalk
+    if _global_jtalk is None:
+        _lazy_init()
+    if not exists(path):
+        raise FileNotFoundError("no such file or directory: %s" % path)
+    if dn_mecab is None:
+        dn_mecab = OPEN_JTALK_DICT_DIR
+    r = _mecab_dict_index(dn_mecab, path.encode("utf-8"), out_path.encode("utf-8"))
+
+    # NOTE: mecab load returns 1 if success, but mecab_dict_index return the opposite
+    # yeah it's confusing...
+    if r != 0:
+        raise RuntimeError("Failed to create user dictionary")
+
+
+def update_global_jtalk_with_user_dict(path):
+    """Update global openjtalk instance with the user dictionary
+
+    Note that this will change the global state of the openjtalk module.
+
+    Args:
+        path (str): path to user dictionary
+    """
+    global _global_jtalk
+    if _global_jtalk is None:
+        _lazy_init()
+    if not exists(path):
+        raise FileNotFoundError("no such file or directory: %s" % path)
+    _global_jtalk = OpenJTalk(
+        dn_mecab=OPEN_JTALK_DICT_DIR, userdic=path.encode("utf-8")
+    )
diff --git a/pyopenjtalk/openjtalk.pyx b/pyopenjtalk/openjtalk.pyx
index 96505b8..291311e 100644
--- a/pyopenjtalk/openjtalk.pyx
+++ b/pyopenjtalk/openjtalk.pyx
@@ -9,9 +9,12 @@ np.import_array()
 
 cimport cython
 from libc.stdlib cimport calloc
+from libc.string cimport strlen
 
 from .openjtalk.mecab cimport Mecab, Mecab_initialize, Mecab_load, Mecab_analysis
 from .openjtalk.mecab cimport Mecab_get_feature, Mecab_get_size, Mecab_refresh, Mecab_clear
+from .openjtalk.mecab cimport createModel, Model, Tagger, Lattice
+from .openjtalk.mecab cimport mecab_dict_index as _mecab_dict_index
 from .openjtalk.njd cimport NJD, NJD_initialize, NJD_refresh, NJD_print, NJD_clear
 from .openjtalk cimport njd as _njd
 from .openjtalk.jpcommon cimport JPCommon, JPCommon_initialize,JPCommon_make_label
@@ -116,18 +119,52 @@ cdef feature2njd(_njd.NJD* njd, features):
         _njd.NJDNode_set_chain_flag(node, feature_node["chain_flag"])
         _njd.NJD_push_node(njd, node)
 
+# based on Mecab_load in impl. from mecab.cpp
+cdef inline int Mecab_load_with_userdic(Mecab *m, char* dicdir, char* userdic):
+    if userdic == NULL or strlen(userdic) == 0:
+        return Mecab_load(m, dicdir)
+
+    if m == NULL or dicdir == NULL or strlen(dicdir) == 0:
+        return 0
+
+    Mecab_clear(m)
+
+    cdef (char*)[5] argv = ["mecab", "-d", dicdir, "-u", userdic]
+    cdef Model *model = createModel(5, argv)
+
+    if model == NULL:
+        return 0
+    m.model = model
+
+    cdef Tagger *tagger = model.createTagger()
+    if tagger == NULL:
+        Mecab_clear(m)
+        return 0
+    m.tagger = tagger
+
+    cdef Lattice *lattice = model.createLattice()
+    if lattice == NULL:
+        Mecab_clear(m)
+        return 0
+    m.lattice = lattice
+
+    return 1
+
 
 cdef class OpenJTalk(object):
     """OpenJTalk
 
     Args:
         dn_mecab (bytes): Dictionaly path for MeCab.
+        userdic (bytes): Dictionary path for MeCab userdic.
+            This option is ignored when empty bytestring is given.
+            Default is empty.
     """
     cdef Mecab* mecab
     cdef NJD* njd
     cdef JPCommon* jpcommon
 
-    def __cinit__(self, bytes dn_mecab=b"/usr/local/dic"):
+    def __cinit__(self, bytes dn_mecab=b"/usr/local/dic", bytes userdic=b""):
         self.mecab = new Mecab()
         self.njd = new NJD()
         self.jpcommon = new JPCommon()
@@ -136,7 +173,7 @@ cdef class OpenJTalk(object):
         NJD_initialize(self.njd)
         JPCommon_initialize(self.jpcommon)
 
-        r = self._load(dn_mecab)
+        r = self._load(dn_mecab, userdic)
         if r != 1:
           self._clear()
           raise RuntimeError("Failed to initalize Mecab")
@@ -147,8 +184,8 @@ cdef class OpenJTalk(object):
       NJD_clear(self.njd)
       JPCommon_clear(self.jpcommon)
 
-    def _load(self, bytes dn_mecab):
-        return Mecab_load(self.mecab, dn_mecab)
+    def _load(self, bytes dn_mecab, bytes userdic):
+        return Mecab_load_with_userdic(self.mecab, dn_mecab, userdic)
 
 
     def run_frontend(self, text):
@@ -231,3 +268,18 @@ cdef class OpenJTalk(object):
         del self.mecab
         del self.njd
         del self.jpcommon
+
+def mecab_dict_index(bytes dn_mecab, bytes path, bytes out_path):
+    cdef (char*)[10] argv = [
+        "mecab-dict-index",
+        "-d",
+        dn_mecab,
+        "-u",
+        out_path,
+        "-f",
+        "utf-8",
+        "-t",
+        "utf-8",
+        path
+    ]
+    return _mecab_dict_index(10, argv)
diff --git a/pyopenjtalk/openjtalk/mecab.pxd b/pyopenjtalk/openjtalk/mecab.pxd
index bd367c7..1538e05 100644
--- a/pyopenjtalk/openjtalk/mecab.pxd
+++ b/pyopenjtalk/openjtalk/mecab.pxd
@@ -16,3 +16,14 @@ cdef extern from "mecab.h":
     char **Mecab_get_feature(Mecab *m)
     cdef int Mecab_refresh(Mecab *m)
     cdef int Mecab_clear(Mecab *m)
+    cdef int mecab_dict_index(int argc, char **argv)
+
+cdef extern from "mecab.h" namespace "MeCab":
+    cdef cppclass Tagger:
+        pass
+    cdef cppclass Lattice:
+        pass
+    cdef cppclass Model:
+        Tagger *createTagger()
+        Lattice *createLattice()
+    cdef Model *createModel(int argc, char **argv)
diff --git a/tests/test_data/.gitignore b/tests/test_data/.gitignore
new file mode 100644
index 0000000..d6b7ef3
--- /dev/null
+++ b/tests/test_data/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/tests/test_openjtalk.py b/tests/test_openjtalk.py
index 0f43363..adfc24e 100644
--- a/tests/test_openjtalk.py
+++ b/tests/test_openjtalk.py
@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import pyopenjtalk
 
 
@@ -80,3 +82,29 @@ def test_g2p_phone():
     ]:
         p = pyopenjtalk.g2p(text, kana=False)
         assert p == pron
+
+
+def test_userdic():
+    for text, expected in [
+        ("nnmn", "n a n a m i N"),
+        ("GNU", "g u n u u"),
+    ]:
+        p = pyopenjtalk.g2p(text)
+        assert p != expected
+
+    user_csv = str(Path(__file__).parent / "test_data" / "user.csv")
+    user_dic = str(Path(__file__).parent / "test_data" / "user.dic")
+
+    with open(user_csv, "w", encoding="utf-8") as f:
+        f.write("ｎｎｍｎ,,,1,名詞,一般,*,*,*,*,ｎｎｍｎ,ナナミン,ナナミン,1/4,*\n")
+        f.write("ＧＮＵ,,,1,名詞,一般,*,*,*,*,ＧＮＵ,グヌー,グヌー,2/3,*\n")
+
+    pyopenjtalk.mecab_dict_index(f.name, user_dic)
+    pyopenjtalk.update_global_jtalk_with_user_dict(user_dic)
+
+    for text, expected in [
+        ("nnmn", "n a n a m i N"),
+        ("GNU", "g u n u u"),
+    ]:
+        p = pyopenjtalk.g2p(text)
+        assert p == expected