Abstract: Objective : To investigate MDR reversing and apoptosis inducing effects treated with arsenic trioxide (As₂O₃ ) in gastric cancer cell line SGC7901/ADM, which appears multidrug resistance. Methods : Immunocytochemistry (PY-method) was used to detect the different expression level of Pglycoprotein (P-gp,mdr-1 gene product), and flow cytometry (FCM) was used to check cell cycle, treated with or without different concentrations of As₂O₃ in SGC -7901/ADM cell line. Results : Reversing effect of Pgp expression was found in SGC7901/ADM cell line when treated with different concentrations of As₂O₃, ranging from 0.1μmol/L to 0.75μmol/L, and cell cycle was blocked at the period of G₀-G₁, augmented with the increasing concentration of As₂O₃ (P<0.01). Conclusions : Arsenic trioxide may play a role in the regulation of MDR reversing and apoptosis induced by the way of dose-dependent in gastric cancer.