This article is about the importance of the analysis of one morpheme pre-final ending `-0-` in Middle Korean. This research suggests that the extent of morphological distribution of `-0-` is in front of the verbal ending, `-n, -1, -m` and the syntactic function of `-0-` is the pre-final ending to construct the NP-embedded sentence in middle Korean. The nominalization by, `- m` and the adnominalization by `- n, -I` can be described together with its same syntactic function to construct NP-embedded sentences, and the obligatory appearance of `-0-` in NP-embedded sentences by nominal construction gives justification to describe `-on, - 01, - om` as a same criterion. And also the disappearance of `-0-` can be explained by the change of the syntactic function. `-0-` is getting disappeared from the NP-embedded sentence construction whose nominality is weakened. It tells that `- 0-` is deeply concerned about the nominality in its syntactic function. The study about the prefinal ending `-0-` in Korean still has many questions in its syntactic function and its disappearance in language change. For this, this article emphasizes the importance of the analysis of one morpheme and suggests very strict criterion.