Abstract： Objective The study was purposed to investigate the synergistic antitumour effects of sodium valproate (VPA) and As2O3 on the proliferation, differentiation and cell cycle of Moh-4 cell in vitro.Methods Cell viability and growth curve were assessed by the MTT assay and the DNA content of Molt-4cell was analyzed by flow cytometry after exposure 36 h to VPA and As2O3. Results The results indicated that the changes of the growth curve and MTT assay showed their inhibition of proliferation to Moh-4 cells.The 50 % inhibition rate of VPA and As2O3 were 4.904 mmol/L and 9.925 μmol/L respectively. There was an addition effect with the combination of the two drugs (values of Q ＞0.85). The inhibition rate was up to (70.31±2.54)% when exposing to 5.0 mmol/L VPA combinating with 10.0 μmol/L As2O3. Flow cytometry showed that the percentage of Molt-4 ceils in G1 phase increased from 28.21% to 71.89 % and the cells in S phase decreased from 71.75 % to 9.49 % after 36 h culture of VPA. Conclusion It was found that the combinations of VPA and As2O3 for 48 h increased inhibition rate to a greater extent than occurred for cell treated with either compound alone. The mechanism needed to be further studied.