The School of Mind created by Lu Jiuyuan, together with the School of Principle of Cheng Brothers and Zhu Xi, has put great impact upon academic, culture as well as society in China. By taking Onto-Mind as the core of Lu's philosophy, this paper examines its spirit of freedom.The rise of Lu's theory of Onto-Mind had its particular background both in practice and in theory. During the reigns of Emperor Gaozong and Emperor Guangzong of the South Song Dynasty, when Lu Jiuyuan lived, the political turbulence, awful administration and unsatisfying custom had made it become the pressing issue how to rectify human mind and justify rites and norms. It was to solve this issue that Lu took the moral of individuals as the foundation of his theory. In addition, Lu's philosophy couldn't be fully understood if we ignore the development of New-Confucianism. In the great synthesis of New-Confucianism in Zhu Xi, some innate theoretical difficulties in Principles of Heaven (tian li 天理) and other issues had stood out and it was in the discourse with Zhu Xi that Lu developed his philosophy. In spite of the fact that Lu had absorbed some elements from Buddhism and Daoism, his philosophy was essentially rooted in Mencius rather than in the Chan School of Buddhism.A mass of discussion on mind could be found in ancient Chinese philosophy. For instance, Mencius emphasized the character of morality in mind while Xunzi that of intelligence; Zhuangzi stressed the subjectivity of mind while Buddhism understood mind from the perspective of ontology. All these theories, especially Zhu Xi's discussion on Mind and Nature had influenced Lu Jiuyuan. Although Lu didn't define the category Mind, it could mainly be understood as Onto-Mind in such basic propositions as "My mind is the universe and vice verse " and "Mind is Principle". Based upon the idea of Onto-Mind, Lu Jiuyuan established his theoretical framework of Onto-Mind—principles of matter—Onto-Mind in his philosophy with the spirit of freedom.As a key category in the Western philosophy, it roughly contains freedom in the political sense and that in the philosophical sense. While the former is not easily found in ancient China due to the absence of right, the latter has been developed in its particular way. Confucianism takes moral freedom as the ultimate aim while it sometimes also purses spiritual freedom advocated by Buddhism and Daoism. Both moral freedom andspiritual freedom could be found in Lu's philosophy. The rational side of mind is achicved by developing Onto-Mind from its original state. Then such Onto-Mind is embodied in principles of matter so that moral deeds are naturally realized in the unity of self-consciousness and self-volition. In this way moral freedom is achieved. In addition, Onto-Mind will turn back from principles of matter and hence realize spiritual freedom since it is free of the burden of life.However, Lu Jiuyuan, just as Zhu Xi that he criticize, does have its own theoretical difficulties. The tension between individual dimension and universal dimension of mind and that between moral freedom and spiritual freedom, which are innate in Lu' theory, could be seen more clearly in Lu's disciples including Yang Jian.