Yin and Yang are a general idea to understand phenomenon and human body. Specially in medicine, the principle of Yin and Yang is clinical and positive. As the principle of Yin and Yang, the human body can be seperated two parts; male-female, upward-downward, front-rear, left-right, interior-exterior, inside-outside and heat-cold, input-output, contraction-relaxation, hard-soft, active-pasive, weekness-strong and old-young, white-black, fat-lean, high-low, long-short, spirit-body. But, Yin and Yang are facing and interchanging each other. Summaries of a study on the means of yin and yang in Naekyoung and Waekyung chapter of DongEuiBoKahm are as follows. 1. Yin and Yang represent Objects . They represent natural phenomena; for example the situation of weather and time is expressed by them. . They represent urinary organ, anus, genital organ. . They represent the five viscera and the six entrails and their functions. . They represent male and female. . They represent fat and lean. 2. Activity and Inactivity are represented by Yin and Yang. ；They mean increasing of temperature by activity and decreasing of temperature by inactivity. . They mean the movement and activity of Ki 3. Yin expresses formation and Yang expresses functions. . Ki and it's functions are expressed by Yin and Yang. . Body fluid, blood and fundamental nutrition are expressed by Yin and Yang ；Organic system of human body and functions of organs are expressed by Yin and Yang. 4. The directions in human body and direction of Ki are expressed by Yin and Yang ；Inside and Outside of human body are expressed by Yin and Yang. . Emission and contraction is expressed by Yin and Yang. ․Upper and lower side of human body are separated by Yin and Yang. ․Upward and downward flow of Qi is expressed by Yin and Yang. . They represent KyungNak, YoungKi, WeKyung.a