Melanin is specifically produced in melanocytes. The pathway for melanin biosynthesis is mainly controlled by tyrosinase. To estimate the inhibitory effect of melanin biosynthesis from 31 medicinal plants extracts, we tested the inhibitory effects of the tyrosinase promoter on B16 mouse melanoma cells. The result of this study demonstrated that Mori Radicis Cortex and Castena Fractus extracts only in tested medicinal plant extracts have high inhibitory effects on tyrosinase promoters with very low cytotoxicity on B16 mouse melanoma cells. Therefore, extracts of Mori Radicis Cortex and Castena Fractus were evaluated as very effective negative regulators of tyrosinase gene expression.