Abstract: Objective To investigate the antitumor effect of MS-275,a histone deacetylase inhibitor,on growth of the human myeloma cell line U266. Methods Cultured U266 cells were exposed to different concentrations of MS-275 for different periods of time. Cell viability was evaluated by trypan blue exclusion assay and cell count. Cell morphologic changes were observed with Wright-Giemsa staining. Cell cycle was analyzed by flow cytometry,and proteins of poly（ADP-ribose） polymerase（PARP）, P21 and Cdk4,were detected by Western blot. Results MS-275 inhibited the growth of U266 cells in a dose-and time-dependent manner. The cell cycle was arrested at G0/G1 phase,and the visible morphological changes in U266 cells were confirmed with Wright-Giemsa staining. Western blot showed cleaved-PARP,increased P21 expression,and down-regulation of Cdk4 expression in MS-275-treated U266 cells. Conclusion MS-275 can inhibit the proliferation of U266 cells,arrest the cell cycle at G0/G1 phase,and induce apoptosis of U266 cells.