Traditional medical ethics which underpins its basis on ‘four principles’ is not sufficient for guiding doctors in complex medical situation. This paper explores Dasan (茶山) Jeong Yakyong’s thought about the concept of Mercy (慈) in Mokminsimseo (牧民心書) that may work as a guidance for modern medi-cine. Firstly, we try to understand Dasan``s thought and the keynote of Mokminsimseo (牧民心書). Nextly, this paper explain the meaning of what is Dasan (茶山) Jeong Yakyong’s the concept of Mercy (慈). Final-ly, we emphasize that Jeong Yakyong’s the concept of Mercy (慈) can be the principle of doctor``s profes-sionalism.