Since Confucius presentied ‘zhong-yong’ and Zi Si wrote Zhong-Yong (The Doctring of the Mean), specially since Zhu Xi edited Zhong-Yong as one of Si-Shu (The Four Books) and interpreted it, zhong-yong was not only recognized as the extreme of morality but also as a significant category of Confucianism. The purpose of this paper is to clarify how Jeong Yak-Yong criticized Zhu Xi s interpretation of zhong-yong, and furthermore, to search how Jung Yak-yong explain the zhong-yong by tree concepts of ‘grasp’, ‘moral behavior’, and ‘habit’. Zhu Xi interpreted zong as a measured absolute middle of two-side and a non-launched original nature. Interpreted yong as a common truth. Therefore he interpreted zhong-yong as a common truth of neither excessive nor enough. Accordingly, Zhu Xi s zhong-yong can be understood as absolute zhong-yong without human’s reflection and moral behavior. But Jeong Yak-Yong interpreted zhong as the state of a man’s very hard-concentrate and grasp of situation, and interpreted yong as the state of a man’s very hard-effort moral behavior and it’s habit. Therefore he explained zhong-yong as a habit of moral behavior through grasp.