Objective To study the effects of arsenic trioxide on the tyrosinase activity and melanogenesis in human malignant melanoma cell line A375. Methods MTT was used to measure the proliferation of human malignant melanoma cell A375. The tyrosinase activity and the melanin content were measured. Results When the concentration of arsenic trioxide was lower than 0.01 ?mol/L, a positive effect was shown along with the increase of the concentration, such as cell proliferation, tyrosinase activity and up-regulation of melanogenesis. When the concentration of arsenic trioxide was above 0.01 ?mol/L, an inhibitory effect was observed. If the concentration of arsenic trioxide was above 1?mol/L, cytotoxic effect occurred. Conclusion Arsenic trioxide has diphasic effect on human malignant melanoma cell line A375.