Genuineness is one of the most important concepts in Confucian philosophy of morality. In the texts before the Spring-Autumn period, the word ”cheng” was usually utilized as an auxiliary word, while in Meng-zi ”cheng” already took on the meaning of morality. In Meng-zi, ”cheng” means genuineness, sincerity and truthfulness. Meng-zi associated ”cheng” with the moral cultivation of human beings. On the one hand, ”cheng” could be taken as the moral principle of nature and society; on the other hand, it is the greatest happiness to reflect on oneself and achieve ”cheng” sincerely. Through the discipline of ”thinking about cheng” and ”complete cheng,” one can reach the realm of the ”heavenly way” and ”cheng.” In The Doctrine of the Mean, the ”heavenly way” is ”genuineness” and ”truthfulness,” and the ”human way” is to understand the inherent good that one is bestowed by the heaven, face oneself truthfully and eliminate the selfish desire through moral cultivation, so as to understand thoroughly through ”genuineness the heavenly way and the human way.” This paper is to understand concisely the thought and value of ”genuineness” in Meng-zi through the following sections: first, the origin of ”genuineness”; second, ”genuineness” and ”good” in Meng-zi; third, ”genuineness” and ”dynamics” in Meng-zi; fourth, ”genuineness” and ”jue” (feeling) in Meng-zi; fifth, the reflection of Meng-zi and The Doctrine of the Mean on ”genuineness”; and finally an statement on the value of ”genuineness” from Meng-zi.