From a diachronic viewpoint, the Chinese language has had a great influence on the characters, phonology and vocabulary of the Japanese language, but its impacts on Japanese grammar are often overlooked in academic circles, especially in terms of the parts of speech of Sino-Japanese vocabulary. The incorporation of Sino-Japanese parts of speech was very significantly affected by the characteristics of Japanese grammar, as Chinese loanwords had to be firstly introduced into Japan in noun form, and were then transformed into other forms only according to the dictates of Japanese grammar. This view springs from Yoshio Yamada’s “A Study of Sino-Japanese vocabulary items.” In the reception of this work, the theory of grammar it was based on was neglected by subsequent scholars, which led to a one-sided and misguided view on Sino-Japanese parts of speech. In the first half of this article, the author raises doubts about the dominant academic view, and illustrates how ancient Chinese loanwords differ from other loanwords of foreign origin, as the former were able to retain their original grammatical characteristics without being transformed into the noun form when they were introduced. In the second half of this article, the background and the reasons for this are analyzed through an interpretation of Yoshio Yamada’s standpoint. In conclusion, this article emphasizes the necessity of understanding the theoretical basis of previous studies instead of passively accepting their conventional perspective. It also points out the necessity of conducting a comprehensive assessment of Sino-Japanese parts of speech through the morphological and syntactic functions and meanings of words.