Since the days of early Confucianism, the nature(性) and the emotion(情) have been the methodological tool to explain the original nature of the human being, by dividing the human mind into the nature and the emotion. This study examines the interconversion(相成相資) theory of the nature and the emotion mentioned in the "Sadanron(四端論)" chapter of "Dong-uisusebowon." We have contemplated how Gi(氣) of sadness and anger mutually create each other, that of pleasure and enjoyment do as well, transforming sad nature[哀性] of the Taeyang-in(太陽人) into anger emotion, anger nature of the Soyang-in(少陽人) into sad emotion[哀情], pleasure nature[喜性] of the Taeuemin(太陰人) into enjoy emotion[樂情], enjoy nature[樂性] of the Soeumin(少陰人) into pleasure emotion[喜情] considering the circumstances in which the nature changes into the emotion based on the original texts of "Dong-uisusebowon", "Sasang-uihakchobon-gwon(四象醫學草本卷)" and "Gyeokchigo(格致藁)".