Objective: To investigate the effects of various concentrations As 2O 3 on malignant melanoma. Methods: The viability of Cloundman melanoma S91 cells treated with As 2O 3 was measured by MTT [3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide] assay. The apoptosis was determined by transferase-mediated dUTP nick end labeling (TUNEL) detection, and the morphology of apoptotic cells was observed through transmission electron microscopy (TEM). The cells growth phase were analyzed by flow cytometry (FCM). Results: A time-and dose-dependent decrease in cell viability was induced in S91 cells after treatment with As 2O 3 at the concentration of 1-5 μmol/L respectively. The TUNEL indices were 0.033±0.018, 0.062±0.012, 0.102±0.016, 0.132±0.031, and 0.162±0.027 respectively, which were much higher compared with the control group (0.017±0.004, P<0.01). The flow cytometry showed that hypodiploid peak after treatment with 3 μmol/L and 5 μmol/L of As 2O 3 for 48 h were 9.99% and 17.59% respectively, which increased significantly compared with the control cells (3.05% P<0.01,). The apoptotic morphology observed by transmission electron microscope showed the chromatin became condensed and attached to the inner surface of nuclear membrane. Conclusion: Arsenic trioxide can induce melanoma Cloundman S91 cell apoptosis at the concentration of 1-5 μmol/L, which will provide enhanced benefit in melanoma therapy. Methods: The viability of Cloundman melanoma S91 cells treated with As 2 O 3 was measured by MTT [3- (4,5-dimethyl-2-thiazolyl) -2 The apoptosis was determined by transferase-mediated dUTP nick end labeling (TUNEL) detection, and the morphology of apoptotic cells was observed through transmission electron microscopy (TEM). The cells growth phase were analyzed by flow cytometry (FCM). Results: A time-and dose-dependent decrease in cell viability was induced in S91 cells after treatment with As 2 O 3 at the concentration of 1-5 μmol / L respectively. The TUNEL indices were 0.033 ± 0.018 ± 0.012, 0.102 ± 0.016, 0.132 ± 0.031, and 0.162 ± 0.027 respectively, which were much higher than the control group (0.017 ± 0.004, P <0.01). The flow cytometry showed that hypodiploid peak after treatment with 3 μmol / L and 5 μmol / L of As 2 O 3 for 48 h were 9.99 % and 17.59% respectively, which increased significantly compared with the control cells (3.05% P <0.01,). The apoptotic morphology observed by transmission electron microscope showed the chromatin condensed and attached to the inner surface of nuclear membrane. Conclusion: Arsenic trioxide can induce melanoma Cloundman S91 cell apoptosis at the concentration of 1-5 μmol / L, which will provide enhanced benefit in melanoma therapy.