Caesalpinia sappan L. has long been commonly used as emmenagogue, analgesic, and a cure for contusion and sprain as well as a remedy for thrombosis in the Oriental medicine. The main constituent of C. sappan is brazilein, which is an antioxidative substance that has a flavonoid structure. In this study, we examined the effect of butanol extract of C. sappan on proliferation and melanogenesis in B16/F10 melanoma cells. After 48h treatment of cells with various concentrations of butanol extract, the cells exhibited a dose-dependent inhibition in their proliferation without apotosis. Therefore, the growth retardation by the extract may be due to the cell arrest, not due to the cell death induced by cytotoxicity. We also estimated total melanin contents as a final product and activity of tyrosinase, a key enzyme, in melanogenesis of B16/F10 melanoma cells. Our result showed that the melanin contents and tyrosinase activity were decreased in butanol extract-treated cells in a dose dependent manner compared to control group. In conclusion, it was observed that butanol extract of C. sappan inhibited melanization of these cells and therefore butanol extract could be developed as skin whitening components of cosmetics.