Genistein, kind of soy isoflavones, is well-known as natural ingredients and consumed as health foods and supplements. They are expected to improve renal function. They have high-affinity to estrogen receptor β expressed predominantly in bone tissue, they prevent osteoporosis specifically and safely. We examined whether genistein can be a new direct capping agent. In this study, we examined the effect of genistein for the proliferation and differentiation of rat dental pulp cells in vitro and the ability of tertiary dentin formation in vivo. As a result, rat dental pulp cells with genistein were increased activity of ALPase and showed alizarin red positive-staining. Calcification-related genes expression has been confirmed by the addition of genistein. From in vivo study, high quality of tertiary dentin formation and minor pulp reaction were observed. From these findings, it was suggested that genistein may be useful agent for direct pulp capping.