1. The purpose of this study is to try and establish the grammatical system of the Koryo language through examining the linguistic facts shown in Kerimyusa, which is thought to reveal the characteristics of the Koryo language substantially and objectively as it was written by a Chinese. The grammatical differences beteen written and spoken languages at that time will be shown; the former is based on Koryo Kayo, and the latter on Kerimyusa. Ultimately this is a preliminary work for setting up the general grammatical system of the Koryo language. 2. The Koryo language had its base in the southern Silla language, was enriched by the nothern Kokuryo language, and had some borrowing from the Mongolia language. This period showed a dual phenomenon between written and spoken languages, consequently a deformative bilingualism. The general linguistic characteristics at that time are as follows: a series of fortis sounds emerged, affricates were pronounced as dental voiceless [ts] and voiced [dz], there was no word-initial consonant cluster formed, and /k, p, s/ were weakened or deleted between vowels or open vowels. 3. The grammatical phenomena revealed in Kerimyusa are as follows: 1) In general no case form was realized. However the nominative case was undergoing a transition from deep to surface cases. 2) The predicate final ending was not used in spoken expression. Therefore the verbal stem functioned as a free form. However, in the primary expressions strong in emotion, interrogative final endings `-箇` and `-易成`, imperative final endings `-라` (humble) and `受勢~少時` (honorific) were used. 3) `-아-/-어-` and `-습-` were used as inflectional pre-final endings. 4) In view of word-formation, derivation and compounding were performed; derivation was confined in suffixation, and compounding in syntactic compounding. Among the stems used in suffixation-`-ol,-ㄴ,-ㅁ,-지`, `-이, ㅁ, -지` were combined with verb stem, and only `-ㄴ` was combined with adjective stem. `-ㄴ` was a multi-functional morpheme; in addition to its function as a derivational stem, it also had its original predicate function as an adjective and the function of verbal determiner. And `-지` was an allomorph of `-ㅁ`. The forms of compounding were `noun+noun` and `noun+adjective`. ?5) In the category of adverbs, an adverb of degree `得` and adverbs of negation `安里` and `沒` were used. The adverbs of negation `安里` and `沒` were mutually interchangeable, which is an important key to investigating the development of negative construction. And the grammatical function of adverbs at that time was equivalent to that of nouns.