Chan-jan's(湛然) article Ten Non-dual Doors (<十不二門>)is an extremely concise essay. It Contains Master Chih-yih's(智顗)doctrines such as the ten wonders of the "phenomenal"door and ever "noumenal"door in Fa Hua Hsuen Yih(《法華玄義》), the concept of "trilicosm in one thought" in Mo Ho Chih Kuan(《摩訶止觀》), ect. Based on these doctrines, it develops into the foundation of Chan-jan's thought and method of meditation. From this, one may understand that Ten Non-dual Doors covers a very wide and profound area, and consists of Tien-tai School's doctrine and meditation method. Also, one may know well the importance of the article in Chan-jan's thought. Only because of the particular importance of this article, the different masters of Tiem-tai School in Sung Dynasty paid very much attention to it. They wrote commentaries on it. Now, six commentaries still exist although many have disappeared. The different commentaries hold many different views. Even confrontation (for example, between "shan chia" [山家] and "shan Wai" [山外])emerged and Jen-yueh (仁岳) rebuted his master Chih-lih's (知禮)argument. Such examples show the impact of Ten Non-dual Doors on the Tiem-tai sudies of Sung Dynasty as well as the its plurality and richness. Although there are many factors which lead to the dispute between "shan chia" and "shan wai", the key point is probable that Chan-jan used the concept of "Trilicosm in one thought" to interpret Tien-tai Master Chih-yi's thought. In particular, Ten Non-dual Doors is usually the object of dispute. In this context, this paper Ten Non-dual Doors to investigate the situation of Tien-tai studies in T'ang and Sung Dynasties. This paper can be divided into three parts. The first part analyzes the contents of the Ten Non-dual Doors. The second part discusses the Ten Non-dual Doors from the standpoint of Chih-yi's Fa Hua Hsuen Yi. The third part deals with the impact of the Ten Non-dual Doors on the "san chia"and "san wai"in Sung Dynasty.