diff --git a/ext/opencv/mat.cpp b/ext/opencv/mat.cpp
index e5b8a77..7f34c61 100644
--- a/ext/opencv/mat.cpp
+++ b/ext/opencv/mat.cpp
@@ -716,6 +716,26 @@ namespace rubyopencv {
       return mat2obj(retptr, CLASS_OF(self));
     }
 
+    VALUE rb_diag(int argc, VALUE *argv, VALUE self) {
+      VALUE d;
+      rb_scan_args(argc, argv, "01", &d);
+      int d_value = NIL_P(d) ? 0 : NUM2INT(d);
+      cv::Mat* selfptr = obj2mat(self);
+      cv::Mat* retptr = NULL;
+
+      try {
+	retptr = new cv::Mat();
+	cv::Mat tmp = selfptr->diag(d_value);
+	tmp.copyTo(*retptr);
+      }
+      catch (cv::Exception& e) {
+	delete retptr;
+	Error::raise(e);
+      }
+
+      return mat2obj(retptr, CLASS_OF(self));
+    }
+
     /*
      * Sets all or some of the array elements to the specified value.
      *
@@ -858,7 +878,8 @@ namespace rubyopencv {
       rb_define_method(rb_klass, "-", RUBY_METHOD_FUNC(rb_sub), 1);
       rb_define_method(rb_klass, "*", RUBY_METHOD_FUNC(rb_mul), 1);
       rb_define_method(rb_klass, "/", RUBY_METHOD_FUNC(rb_div), 1);
-      
+      rb_define_method(rb_klass, "diag", RUBY_METHOD_FUNC(rb_diag), -1);
+
       rb_define_method(rb_klass, "clone", RUBY_METHOD_FUNC(rb_clone), 0);
 
       rb_define_method(rb_klass, "rows", RUBY_METHOD_FUNC(rb_rows), 0);
diff --git a/test/test_mat.rb b/test/test_mat.rb
index db5f2b3..8975617 100755
--- a/test/test_mat.rb
+++ b/test/test_mat.rb
@@ -322,6 +322,37 @@ class TestMat < OpenCVTestCase
     }
   end 
 
+  def test_diag
+    m0 = Mat.new(3, 3, CV_8U)
+    i = 1
+    m0.rows.times { |r|
+      m0.cols.times { |c|
+        m0[r, c] = Scalar.new(i)
+        i += 1
+      }
+    }
+
+    m1 = m0.diag
+    elems = m1.to_s.scan(/(\[[^\]]+\])/m).flatten[0]
+    assert_equal("[  1;\n   5;\n   9]", elems)
+
+    m2 = m0.diag(0)
+    elems = m2.to_s.scan(/(\[[^\]]+\])/m).flatten[0]
+    assert_equal("[  1;\n   5;\n   9]", elems)
+
+    m3 = m0.diag(1)
+    elems = m3.to_s.scan(/(\[[^\]]+\])/m).flatten[0]
+    assert_equal("[  2;\n   6]", elems)
+
+    m4 = m0.diag(-1)
+    elems = m4.to_s.scan(/(\[[^\]]+\])/m).flatten[0]
+    assert_equal("[  4;\n   8]", elems)
+
+    assert_raise(TypeError) {
+      m0.diag(DUMMY_OBJ)
+    }
+  end
+
   def test_cvt_color
     m = Mat.new(1, 1, CV_32FC3)
     m[0, 0] = Scalar.new(1.0, 2.0, 3.0)