Abstract: AIM To investigate the effect of arsenic trioxide (As2O3) on cellular morphology and proliferation of endothelial cells and to evaluate its value in inhibiting restenosis. METHODS Human umbilical endothelial cells (ECV-304) were incubated with As2O3 in vitro (0 μmol/L, 0.01 μmol/L, 0.1 μmol/L, 1 μmol/L and 10 μmol/L). Cell morphology was observed by inverted microscopy, cell survival number was described by cell growth curve and cell viability was measured by MTT method. RESULTS After incubation, paste wall cells decrease and float and vacuolation were observed in endothelial cells in 10 μmol/L As2O3 group. As2O3 of 0.01-0.1 μmol/L exerted no effect on the A value of endothelial cells. However, the A values of endothelial cells incubated with As2O3 of 1-10 μmol/L were lower than those in control group, indicating that As2O3 inhibited the proliferation of endothelial cells. CONCLUSION As2O3 (10 μmol/L) damages cellular morphology and inhibits cell proliferation.