“Repentance” is one of the major practices in Buddhism. The origin and development of systematized repentance rituals in Chinese Buddhism could be traced back to the four repentance rituals composed by Zhiyi 智顗 (538-597) of Tiantai School in Sui Dynasty. Afterwards, Chengguan 澄觀 (738-839),the Fourth Patriarch in Huayan School, when residing at Da huayan si 大華嚴寺 in Wutai Mountain, was moved and then emphasized the importance of repentance practice as well as practicing the Fangdeng repentance ritual.Nonetheless, what is the content of the Fangdeng repentance ritual conducted by Chengguan? Why the practice of repentance could lead to repenting of crimes, or eventually being liberated? How Chengguan carried out the practice of repentance and what was the specific procedure? Moreover, which grandmasters have contributed to the core essence of his cognition of repentance practice? The aforementioned inquiries all deserve to be clarified and to be pondered on, which constitute the main purpose of this study. The author discovered the followings after further investigation: 1. The system of Chengguan’s repentance thoughts accommodates and integrates a wide variety of schools and doctrines. The influences from doctrines and rituals of schools like Lu (Vinaya), Tiantai, Huayan, and Chan could be found in his teachings and writings. 2. Sources contributing to Chengguan’s repentance thoughts: (1) Tiantai: Bringing to Chengguan how to construct a complete repentance ritual, for example, the ritual of “Ten-dharma” module and so forth. In addition, to integrate the teachings of “Empty nature of crimes” and “Middle way reality” in meditative contemplation of repentance rituals of “Shi 事” and“Li 理”. 澄觀懺悔思想之研究 .67. (2) Huayan: The emphasis on the doctrine of “Five repentance” and theclassification ideas. 3. Major content in Chengguan’s repentance thoughts: (1) Doctrine and practice of Fangdeng repentance ritual, and the emphasis of its dedicated practice. (2) Views on karma of crimes: Five statements like Three-nigha teaching and so on. (3) Views on precepts: Three major viewpoints like the correction of translation of terms from Sanskrit related to disciplines. (4) The essences of repentance: Four essences of repentance practices, e.g., powers from Sutra of “Fangdeng” or from Buddhas or Bodhisattvas. (5) Approaches to repentance: Six approaches to repentance like the repentance by ritual performance, etc. 4. A simplified version of Chengguan’s repentance ritual is introduced in this article. Four features are named from such repentance practice, including “The equal stress on three types of repentance,” and so on. Finally, this research is aiming at the clarification of the real meaning of “Dharma assembly 法會” via discussion and ponderment of Chengguan’s repentance thoughts and rituals, and to follow the spirit of the patriarch’s repentance practice, and, furthermore, to serve as reference to future researchers and practitioners