Since a few Christian manuscripts were found at Tunhuang in last century, the study of Nestorian Christianity of the T'ang Dynasty (AD616-877) had come into a mew era. But, by one hand, it is very difficult to understand the obscure words of these manuscripts, by the other hand, the source of these documents is not easy traced back, so some scholars questioned the authenticity of some of these documents, In this paper, I'll analyse two Chinese Nestorian manuscripts, according to their extrinsic and intrinsic evidences, to see if they are fake documents or not. The two manuscripts are Hsuan yuan chih pen ching and Da sheng tung cheng que fa tsang-which were called by P. Saeki as ”Koshima documents”.