The objective of this study was to investigate the underlying mechanisms behind the radiation-sensitising effects of the antennapedia proteins (ANTP)-smacN7 fusion protein on tumour cells. ANTP-SmacN7 fusion proteins were synthesised, and the ability of this fusion protein to penetrate cells was observed. Effects of radiation on the expression of X-linked inhibitor of apoptosis protein (XIAP) were detected by western blotting. The radiation-sensitising effects of ANTP-SmacN7 fusion proteins were observed by a clonogenic assay. The effects of drugs and radiation on tumour cell apoptosis were determined using Annexin V/FITC double staining. Changes in caspase-8, caspase-9 and caspase-3 were detected by western blot before and after ANTP-SmacN7 inhibition of XIAP. The ANTP-SmacN7 fusion protein could enter and accumulate in cells; in vitro XIAP expression of radiation-induced tumour cells was negatively correlated with tumour radiosensitivity. The ANTP-SmacN7 fusion protein promoted tumour cell apoptosis through the activation of caspase3. ANTP-SmacN7 fusion protein may reduce tumour cell radioresistance by inducing caspase3 activation.