AIM: To determine the effect of allitridi on cell cycle of human gastric cancer (HGC) cell lines MGC803 and SGC7901 and its possible mechanism.METHODS: Trypan blue dye exclusion was used to evaluate the proliferation, inhibition of cells and damages of these cells were detected with electron microscope.Flow cytometry and cell mitotic index were used to analyze the change of cell cycle, immunohistochemistry, and RT-PCR was used to examine expression of the p21WAF1 gene.RESULTS: MGC803 cell growth was inhibited by allitridi with 24 h IC50 being 6.4 μg/mL. SGC7901 cell growth was also inhibited by allitridi with 24 h IC50 being 7.3 μg/mL.After being treated with allitridi at the concentration of 12 μg/mL for 24 h, cells were found to have direct cytotoxic effects, including broken cellular membrane, swollen and vesiculated mitochondria and rough endoplasmic reticula,and mass lipid droplet. When cells were treated with allitridi at the concentration of 3, 6, and 9 μg/mL for 24 h, the percentage of G0/G1 phase cells was decreased and that of G2/M phase cells was significantly increased (P = 0.002)compared with those in the group. When cells were treated with allitridi at the concentration of 6 μg/mL, cell mitotic index was much higher (P = 0.003) than that of control group, indicating that allitridi could cause gastric cancer cell arrest in M phase. Besides, the expression levels of p21WAF1 gene of MGC803 cells and p21WAF1 gene of SGC7901 cells were remarkably upregulated after treatment.CONCLUSION: Allitridi can cause gastric cancer cell arrest in M phase, and this may be one of the mechanisms for inhibiting cell proliferation. Effect of allitridi on cells in M phas e may be associated with the upregulation of p21WAF1 genes. This study provides experimental data for clinical use of allitridi in the treatment of gastric carcinoma.