Abstract: Objective : To study the effect of allicin on human gastric cancer cells lines MGC-803 and SGC-7901 and that combination of allicin with antitumor drugs enhances cytotoxicity of anti-tumor drugs to these cancer cells. Methods : To evaluate the inhibition rates of proliferation of cells bymeans of trypan-blue exclusion, to analyze the change of cell cycle using flow cytometry and Giemsastaining, to study the change of cytotoxicity of BLM A5 and VCR after combination of allicin 3μg/mlwith these cell cycle phase specific antitumor drugs. Results : Allicin inhibited cell growth of MGC-803, IC₅₀ of 24 hours was 6.4μg/ml; Allicin inhibited cell growth of SGC-7901, IC₅₀ of 24 hours was7.3μg/ml. These cells were treated by allicin at the concentrations of 3μg/ml, 6μg/ml, 9μg/ml for 24hours, compared with the control group, the percentage of G₀/G₁, phase of the cells was decreased andthat of G₂/M cells was increased significantly in the allicin treated group (P<0.01). Treated by allicin atthe concentrations of 6μg/ml for 24 hours, compared with the control group, cell division index weremuch higher, it showed that allicin caused arrest of gastricai cancer cells in M phase. In comparisonwith IC₅₀ of BLM A₅ and VCR alone or combined with allicin 3μg/ml, the 24h IC₅₀ values decreasedfrom 1.210μg/ml and 0.125μg/ml to 0.370 and 0.031 N.,ml, respectively in MGC-803 cells; and decreased from 1.760μg/ml and 0.287μg/ml to 0.680μg/ml and 0.074μg/ml,respectively in SGC-7901cells. Conclusions : Allicin caused arrest of gastric cancer cells in M phase, and allicin can enhance cy-totoxicity of the cell cycle phase specific antitumor drugs to these cancer cells.