Abstract: Objective 　To explore the effect s on chemotherapeutic sensitivity of gast ric cancer cell line by stable over2expression of smac gene. Methods 　Under the induction of liposome, the eukaryotic expression vector pcDNA3. 1-smac for smac gene and it s cont rol vector pcDNA3. 1 were t ransfected into gasric cancer cell line MKN-45. The subclone cell lines were obtained by persistent G₄₁₈ selection. smac gene expression of cancer cells were detected by RT-PCR and Western Blot methods. The growth inhibition effect s of mitomycin (MMC) on cancer cells were also observed by tet razolium bromide colorimetry and clone formation test . Results 　The subclone gast ric cancer cell lines, stable expressing smac and neo gene respectively, were successfully selected, named as MKN-45/ smac and MKN-45/ neo. RT-PCR and Western Blot result s demonst rated smac mRNA and protein levels of MKN-45/ smac cells were significantly higher than those of MKN-45 and MKN-45/ neo ( P < 0. 01) . Af ter being t reated with 10 μg/ ml MMC for 24h, the growth inhibition rates of MKN-45 and MKN-45/ neo were 27. 85 %, 28. 12 % respectively, with that of MKN-45/ smac cells being 43. 71 % ( P < 0. 01) . When compared with MKN-45 and MKN245/ neo cells, the clone formation abilities of MKN245/ smac were reduced by 14. 07 % ( P < 0. 01), 15. 13 %( P < 0. 01) respectively. Conclusion 　Stable t ransfection of smac gene and it s over-expression in gastric cancer cell line could significantly improve their chemotherapeutic sensitivities to MMC, which established an experimental basis for ameliorating chemotherapy of gast ric cancer.