This is a study of how some Korean words, such as "garam(??)", "garachida(??치다)", "greuda(그르다)", "geumeul(그믈)", "geusgi(그.기)", "gil(길ㅎ)", "ganalda (??다)", "geureus(그릇)", "gipda(깊다)", changed in their word-forms during the period from the 15 century to the 21 century. The study is based on the aspects of changes in word-forms during this period : ① a death of word-forms, ② a split of word-forms, ③ a change of word-forms, and ④ a retention of word-forms. The study also shows how some words of the 15 century changed in terms of a phoneme and meaning. Word materials in the old Korean books were used for analysis.