Abstract: The motive power of moral action and the growth of Personality are both the important questions in moral philosophy. Mencius considers the sense of compassion (恻隐之心) that is the unmediated, immediateemotional factors as the innate motive power of moral action, and as the initiation of the growth of Personality; Through the theory of practice of Zhiyan and Yangqi (知言养气), Mencius elaborates the progress of the growth of Personality: Zhiyan(知言) is distinguishing good and evil, right and wrong, thus forming the certain belief; Moreover, Yangqi(养气) with the full of morality(集义) as medium affords innate support for the growth of Personality, and through Confucius’ appearance of the saint, Mencius demonstrates the ultimate goal of the theory of moral practice —fulfilling Personality. However, to realize this goal, we must start from the innate emotion Mind of Cannot bear(不忍人之心), and get through the spontaneity of natural instincts of action from RenYi(由仁义行), and fully show innate virtues at external bodies and forms, and make the forms and looks(形色) that are nature of the bodies completelyinternalize the human’s natural instincts, and ascribe all the source of moral action to it, thus accomplish the moral action’s naturalization. That is, by means of “Form Perfect Representation”(践形), the human reaches the saint Personality’s perfection &achievement.