佛經文獻保存了漢語歷史上大量白話口語的語料，對於漢語語法史的研究具有相當重要的價值。本文分析「已」字在中古佛經裏的使用情形，指出「已」字在佛經裏同時具有「副詞」、「謂語動詞」、「結果補語」、「動相補語」、「語氣助詞」、「體貌助詞」等語法功能。其中「副詞」與「語氣助詞」的用法乃承襲上古漢語而來，在佛經語料裏並沒有明顯的變化。「謂語動詞」、「結果補語」、「動相補語」、「體貌助詞」等功能，則呈現出「已」字一連串語法化的過程。 就「已」字語法化的動因而言，它從「謂語動詞」至「結果補語」的演變，乃漢語內部語法體系的發展。「結果補語」擴充至「動相補語」的用法，則是受到佛經梵文翻譯的影響。至於「體貌助詞」的功能，則是在翻譯佛經時，梵漢混合使用所構成的語法成分。

Buddhist scriptures preserved a large mount of oral materials in Chinese history, which has very important value in such study. This article will analyze the usage of ”yi” in medieval Buddhist scriptures and point out that ”yi” has the grammatical functions of ”adverb”, ”predicate verb”, ”result complement”, ”phase complement”, ”particles of speech” and ”aspectual particle” in Buddhist scriptures. The usage of ”adverb” and ”particles of speech” was inherited from the ancient Chinese, but there is no great change in Buddhist scriptures. The functions of ”predicate verb”, ”result complement”, ”phase complement” and ”aspectual particle” have showed a series of grammaticalization process of ”yi”. To the motive of grammaticalization on ”yi”, it has changed from ”predicate verb” to ”result complement”, which is the internal development within Chinese grammar system. The extended usage of ”result complement” to ”phase complement” was influenced by Buddhist scriptures which were translated from Sanskrit to Chinese. As to the function of ”aspectual particle”, it was the grammatical elements formed by mixing the usage of Sanskrit and Chinese when the Buddhist scriptures were translated.