This paper researches the mistake of interchange /p/, /m/ claimed at Seo jae keuk(1964) and on Hyangga ‘執音乎’. The interchange of /p/, /m/ can’t be appeared in present Korean. This interchange of /p/, /m/ was the remains of Ancient Korean. The truth is that Ancient Korean phoneme /b/ had been disappeared and joined Ancient Korean phoneme /p/. This problem can be solved by intimacy of features. And this paper can solve Hyangga ‘執音乎’ which was not settled till now. ‘執音乎’ was decoded on [t am on], because of interchange of /m/, /b/. It also could be interchanged on [t ab on]. This interchange occurs in open syllable frequently like present Japanese. It also occurred in Ancient Korean. It suggests that Ancient Korean was open syllable language.