The expositional framework of "Two-gates-in-one-mind" in The Awakening of Faith in the Mahayana expounds both the ontic status of beings and the radicalization of becoming a Buddha. Furthermore, it constitutes the Buddhist "duplex ontology". Mou Tzun-san treated "Two-gates-in-one-mind" as an important philosophical scheme and deduced "duplex ontology" from it seriously. This paper follows Mou T'zun-san approach to examine the very concept of the one single mind embracing ignorance and dharma-nature simultaneously", the central concept of "hsing-chuyuan- chiao" in T'ien-t'ai Buddhism, anddiscusses the usefulness and universality of "Two-gates-in-one-mind" as a philosophical scheme. This paper discusses also the concrete contents and the special significances of the one single mind embracing ignorance and dharma-nature simultaneously" in T'ien-t'ai Buddhist canon.