The purpose in this paper is two. One is to examine units of educational vocabulary; morpheme, word and lexeme. The other is to seek out the best unit among these three as units of educational vocabulary. A morpheme is a linguistic unit which does not regard synchronic and diachronic one. Therefore it is excluded from an educational vocabulary unit. A word is excluded also, because it has many problems. A definition of the terminology “Lexeme” is two. One is “synchronically a meaningful minimal unit”. The other is “an abstract unit that represents variations of words”. But these concepts are not sufficient. Hence I revise the notion of lexeme and designate lexemeⅢ. It is defined as follows. (1) synchronically a meaningful minimal unit. (2) a unit that includes all of inflections of the root word. (3) an item that functions as a single meaning unit, regardless of the number of words it contains. Excepting secondary questions, lexemeⅢ settles all problems. Consequently, I conclude that lexemeⅢ is suitable as an educational vocabulary unit.