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Background The second mitochondria-derived activator of caspases (Smac) is a novel proapoptotic gene, which plays an important role in the apoptosis-inducing effects of irradiation on tumor cells. The purpose of this study was to investigate the effects of extrinsic Smac gene transfer and its over-expression in radiotherapeutic sensitivities of cervical cancer cells. Methods After the Smac gene was transferred into the cervical cancer cell line HeLa, subcloned cells were obtained by persistent G418 selection. Cellular Smac gene expression was detected by RT-PCR and West blot, while in vitro cell viabilities were detected by trypan blue staining assay. After treatment with X-ray irradiation, cellular radiotherapeutic sensitivities were investigated by tetrazolium bromide colorimetry. Cellular apoptosis and its rate were determined by electronic microscopy, annexin V-FITC and propidium iodide staining flow cytometry. The expression and activities of cellular caspase-3 were assayed by West blot and colorimetry. Results Smac mRNA and protein levels in HeLa/Smac cells and the selected subclone cell line of cervical cancer were significantly higher than those of HeLa (P<0.01). There was no significant difference in cellular viabilities between them (P>0.05). However, after irradiation with 8 Gy X-ray, growth activities of HeLa/Smac were reduced by 22.42% (P<0.01). When compared with those of HeLa, partial HeLa/Smac cells presented characteristic morphological changes of apoptosis under electronic microscope, with higher apoptosis rates (16.4% vs. 6.2%, P<0.01); the caspase-3 expression levels in HeLa/Smac cells were improved significantly (P<0.01), while its activities were increased by 3.42 times (P<0.01).Conclusions Stable transfer of the extrinsic Smac gene and its over-expression in cervical cancer cell line could significantly enhance the expression and activities of cellular caspase-3 and ameliorate apoptosis-inducing effects of irradiation on cancer cells, which was a novel strategy to improve radiotherapeutic effects on cervical cancer.