The purposes of this paper are to reveal necessity and educational usefulness of compound phonemes. For this purpose, the nature of semi-vowels must be revealed first. Originally, so-called semi-vowel is only sound unit handled at the phonetics level, such as glide, approximant, and sliding sound, etc. and semi-vowels are not phoneme, I think. A double vowel is a compound phoneme, and a semi-vowel is just a component of it. When pronounced as one syllable, the vowel that make up one syllable are one phoneme, and just like monophthongs, so-called double vowel is compound vowel and one phoneme. In this way, if the semi-vowel is not recognized as a phoneme but is simply viewed as a slipping sound, educational usefullness increases significantly. Language units can be systematically presented, and the value of Hangeul as a phonemic character can be increased. In Korean language education, instructors and learners can easily conduct speech sound education in an explanatory manner. It is also consistent with the contents of language norms such as Korean spelling and standard pronunciation.