Abstract: Objective: To investigate the effect of arsenic trioxide (As₂O₃) on the cell cycle and to assess whether Skp2 is involved in the downregulation of the cyclin-dependent kinase inhibitor (CDKI) p27^kip1 in As₂O₃-treated human hepatocellular carcinoma (HCC) SMMC-7721 cells. Methods: Cultured in vitro, HCC SMMC-7721 cells were treated for 72h with 2μm ol/L A₂O₃. The percentage of cells in each phase of the cell cycle was detected by flow cytometry (FCM). The expression and localization of p27^kip1 and Skp2 were detected by western blot and immunofluorescence. Results: The SMMC-7721 cell cycle was arrested in G2/M by As₂O₃, compared with the control group. A striking decrease in Skp2 expression and a reciprocal increase in p27^kip1 expression were found in A₂O₃-treated SMMC-7721 cells. Meanwhile, the location of p27^kip1 was transferred from cytoplasm to nucleus in treated cells. Conclusion: A₂O₃ attenuates Skp2 expression, thereby promoting p27^kip1 accumulation in SMMC-7721 cells and inducing cell cycle arrest and growth inhibition.