Objective To study the effect of arsenic trioxide (As_2O_3) on the expression of Fas/FasL and in inducing apoptosis of gastric tumor cells in nude mice. Methods The xenograft model was established in 30 male BALB/C-nu/nu mice. The mice were randomly divided into 3 groups and injected intraperitoneally with As_2O_3 (2.5mg/kg or 5mg/kg) or with saline solution of the same volume respectively. Tumor growth was detected by measuring tumor volume. Apoptosis of the tumor cells was measured with transmission electron microscope (TEM) and TdT-mediated dUTP nick end labeling (TUNEL). The expression of Fas/FasL in the tumor cells was detected with immunohistochemistry. Results Tumor volume in As_2O_3-treated groups was smaller than that in control group (P<0.05). Apoptotic cells in the As_2O_3-treated groups significantly outnumbered than those in control group (P<0.01). The expression of Fas in tumor cells was higher in the As_2O_3-treated groups than in control group (P<0.05), but FasL expression of tumor cells did not differ between the treated groups and control group (P>0.05). Conclusion As_2O_3 can obviously inhibit the growth of human gastric tumor. One of the mechanisms is up-regulation of Fas gene that can induce the apoptosis of gastric cells.