The purpose of this study is to propose the contents of word class conversions by grammaticalization. Word class conversions in grammar education are presented as synchronic phenomenon in which words of the same form are used as different part of speech. Language is a representation of the process and consequences of cognitive thinking by language speakers, and these changes are attributed to comprehensive aspects of history, culture, and cognitive working of language users. Grammaticalization theory is an understanding of linguistic phenomena from a panchronic perspective that encompasses synchronic and diachronic aspects, and it is a more useful methodology to grasp the original characteristics of language. Therefore, as an example of word class conversions in grammar textbooks of 2015 curriculum, the process of grammaticalization was discussed, focusing on the frequently appearing 'mankeum', 'Oneul'. When explained by the process of grammaticalization, learners can clearly identify the property of the part of speech and relate it to the speaker's intention. In addition, the learners can discover that word class conversions are also occur throughout the life of a language and identify the cause and aspect by grammaticalization.