Mind-Nature is the core idea of Han Buddhism, especially Chinese Chan (Zen). Chan was founded by Bodhidharma and then well developed by the Sixth Ancestor, Huineng. Traditionally, Mazu Daoyi (709-788) is depicted as a successor in the lineage of Huineng, since his teacher Nanyue Huairang is regarded as a student and successor of Huineng. Mazu’s teaching of Chan were given to the sentient beings according to their capability. While carrying forward the Sixth Ancestor’s thought of Chan, Mazu also developed it innovatively. Thus, he founded Hongzhou school, which has a profound influence on later generations of Chan practitioners. This paper discusses three aspects of Mazu’s thought of Chan within the frameworks of Essence-Function (Tiyong) and Mind-Nature (Xinxing). It intends to explore the three new ideas of Mind-Nature in Hongzhou school proposed by Mazu: “the truth is in the mind, the truth is in the Buddha” and its corollary－“the truth is not in the mind, the truth is not in the Buddha”, and finally “the calm mind is the Dao”. He developed the thought of Mind-Nature in Chan school, which has become a characteristic of Hongzhou school. Nevertheless, Mazu Daoyi’s Chan thinking faithfully inherits the relevant Chan thought of Huineng. Therefore, the Hongzhou school is considered to be an orthodox branch of Chan propagated by Huineng and his followers.