In order to study the role of BRCA2 protein in homologous recombination repair and radio‐sensitization, we utilized RNA interference strategy in vitro and in vivo with human tumor cells. HeLa cells transfected with small‐interfering BRCA2 NA (BRCA2 siRNA) (Qiagen) as well as negative‐control siRNA for 48 h were irradiated, and several critical end points were examined. The radiation cell survival level was significantly reduced in HeLa cells with BRCA2 siRNA when compared with mock‐ or negative‐control siRNA transfected cells. DNA double strand break repair as measured by constant field gel‐electrophoresis showed a clear inhibition in cells with BRCA2 siRNA, while little inhibition was observed in cells with negative control siRNA. Our immuno‐staining experiments revealed a significant delay in Rad51 foci formation in cells with BRCA2 siRNA when compared with the control populations. However, none of the non‐homologous end joining proteins nor the phosphorylation of DNA‐dependent protein kinase catalytic subunit was affected in cells transfected with BRCA2 siRNA. In addition, the combined treatment with radiation and BRCA2 siRNA in xenograft model with HeLa cells showed an efficient inhibition of in vivo tumor growth. Our results demonstrate down‐regulation of BRCA2 leads to radio‐sensitization mainly through the inhibition of homologous recombination repair type double‐strand break repair; a possibility of using BRCA2 siRNA as an effective radiosensitizer in tumor radiotherapy may arise. (Cancer Sci 2008; 99: 810–815)