This paper aims at the survey of the characteristics of the language and writing system in Koryo dynasty period. Owing to the long time of nearly five hundred years and the lack of convincing materials, we can not describe the systematic explanation such as the sitution of Yi dynasty period. However, we try to find outstanding features of the language and writing system m Koryo dynasty period. We deal with the Chih-lin Lei-shih(鷄林類事) as language data and Kugyol written on the side-line of the pasge on some of Buddhism-Sutra as writing materials. After the detailed discussion, we have reached the following conclusions. 1. Chih-lin Lei-shih shows the early language in Koryo dynasty, and also the first stage of unifying language of Han peninsula. 2. Language materials m Chih-lin Lei-shih are considered as the next generation of Silla common language even though a few materials have Koguryo stratum words. 3. The early stage of Koryo language had two lexical system; pure Korean and Sino Korean. But the later stage had three lexical system: pure, Sino and Mongolian loan words. 4. The writing system had pararelled in two ways; one was the Orthodox Chinese, the other was the borrowing system of Chinese characters adopted to the Korean language (shortly called the Borrowing system). 5. The using of Borrowing system is divided into four categories: Cha-myong(name transcription), Hyang-Chal (poem-transcription), Yi-du (document-transcription) and Kugyol (grammatical-ending transcription). Through the whole period of Koryo-dynasty Yi-du and Kugyol have been the productive writing ways. 6. Especially the simple form of Ku-gyol had been derived from the original form. This simple forms of Ku-gyol is considered to be the primitive model of the Korean alpahbet Hangul in the next dynasty.