This study was performed to investigate the effects of acteoside and isoacteoside isolated from Clerodendron trichotomum Thunberg on melanin production in B16 melanoma cells. In DPPH radical scavenging activity, acteoside and isoacteoside had a potent anti-oxidant activity in a dose-dependent manner. Both acteoside and isoacteoside dose-dependently inhibited silica-induced ROS (reactive oxygen species) generation in B16 melanoma cells. They significantly inhibited tyrosinase activity and melanin production in MSH-stimulated B16 melanoma cells. The inhibitory effect of acteoside was more potent than that of isoacteosidee. In Western blot of tyrosinase, acteoside inhibited MSH-induced tyrosinase expression in B16 melanoma cells, which is related to the inhibitory action of acteoside on tyrosinase activity and melanin production. These results show that acteoside and isoacteoside from Clerodendron trichotomum Thunberg has a potent antioxidant activity and whitening activity. The underlying mechanism of acteoside on whitening activity may be due to the inhibition of tyrosinase activity and tyrosinase expression.