Objective： To observe the mechanisms of imported highly purifi ed As2O3 inhibiting human gastric carcinoma cell lines. Methods： A wide range concentration of As2O3 (0.1-2.0 μmol/L) were cultured w ith gastric carcinoma cell lines SGC7901 and MKN45 with different time, the n, the number of active cells, cell morphology, cell-DNA content distribution were evaluated. NIH3T3 cells were as controls. Results： 1.0-2 .0 μm ol/L As2O3 treated cells presented some typical features of apoptosis such a s intact cell membrane, chromatin condensation, nucleic fragmentation and apopto tic body formation, The analysis of flow cytometery showed that sub-G1 cells immerged before the G1 phase, and show ed a positive correlation with the culture period and concentration of As2O3 . Agrose electrophoresis showed a specific DNA apoptotic ladder. Conclu sion： As2O3 can inhibit human gastric carcinoma cells, the induction of apoptosis is a very important mechanism of As2O3 to treat gastric carcinoma cells.