||The primary concern of Zhuangzi is to achieve oneness with the Tao and to wander freely in this world. In order to achieve this aim, forgetfulness, especially sitting in oblivion (zuowang), is practiced. However, this concept has never been fully developed until the emergence of the doctrine of two-fold mystery in the Sui and Tang dynasty. Particularly in Tang dynasty, the importance of two-fold mystery to sitting in oblivion had been raised and discussed by two renowned Taoists, Cheng Xuan-ying and Si-ma Cheng-zhen. They not only enriched the concept of sitting in oblivion by providing specific guidance, but also advanced the doctrine of Zhuangzi by using the interpretation of abandoning and discarding. This thesis, therefore, aims to analysis the concept of sitting in oblivion in the perspective of two-fold mystery, and reveals the importance of two-fold mystery to the interpretation of sitting in oblivion, with focus on the interpretations by Cheng Xuan-ying and Si-ma Cheng-zhen. A comparison between their interpretations is also discussed in the conclusion of this thesis. In my study, I discussed the interpretation of sitting in oblivion by Cheng Xuan-ying. In his doctrine, sitting in oblivion is interpreted as a spiritual cultivation through a series of abandoning, including the abandoning of virtues, rituals of the society, physical body, knowledge of this world, the boundary between various things and the self, life and death and finally the abandoning of following changes. After all the abandonment, one will reach the state of achieving oneness with the Tao. For Cheng Xuan-ying, the purpose of sitting in oblivion is to forget and discard all the attachments so that one can engage with the ever-changing reality without being harmed. My study also discussed the doctrine of Si-ma Cheng-zhen. In his doctrine, sitting in oblivion is interpreted as a body and mind practice, and is divided into seven steps, including respecting and believing in the Tao, cutting off from the material world, gathering of heart-mind, simplifying one’s daily life, having real observation, reaching great calmness and finally reaching the Tao. By following the practice of these seven steps, one will reach the Tao and at the same time achieve longevity. In the conclusion, a comparison between the two interpretations is discussed. My discussion revealed that while both of them interpreted sitting in oblivion in the perspective of two-fold mystery, Cheng Xuan-ying adopted a philosophical perspective of Taoism while Si-ma Cheng-zhen interpreted this concept in a religious perspective of Taoism.