The meaning of Ch'an Buddhism's theory of "no mind" (無心) has close relations with the contents of Prajna-siltra (般若經). In other words, "no mind" is equivalent to the insight of "emptiness" (空), which is Buddhism's most important and unique concept. Lao Tzu's and Chuang Tzu's theory of "no mind" is related to the recognition of Tao, which is an absolute metaphysical reality. One should keep one's mind calm and empty in order to recognize the significance of Tao. Kuo Hsiang's interpretation of "no mind" is consistent with his distinguished theory of naturalism. Lao Tzu's and Chuang Tzu's "no mind" is always regarded as a means to realize the Tao, while Kuo Hsiang's viewpoint shows another means of access to the Tao, which reflects the characteristics of his era.In general, Ch'an Buddhism and Taoism's theory of "no mind" are regarded as an expression of the tranquility of the mind, and therefore their contents and aims always overlap. Strictly speaking, they have their own theoretical groundings. Through this paper, we could gain a better understanding of their characteristics and increase interest in the study of the essence of the human mind.