In Fu-Xing Shu (Book of Returning to One's True Nature), Li Ao claims that the true nature of all human beings is fundamentally tranquil. However, the Sage's mind is always tranquil while ordinary people are easily disturbed by emotions so that they fail to return to their true nature. Hence Li Ao asserts the principle of tranquility as the key point of his theory of self-cultivation, whose aim is to prevent men from being disturbed by emotions and to make them regain their true nature. Most scholars only see the impact of Buddhism on Li Ao's thought, regardless of the impact of Daoism. In this paper, I argue that Li Ao's principle of tranquility can be regarded as a trace of the interaction of Confucianism and Daoism during the period of pre-Qin to Tang Dynasty.