Tumor residue or recurrence is common after radiation therapy for nasopharyngeal cancer (NPC) since the tumor cells can repair irradiation-induced DNA damage. The ubiquitination cascade mediates the assembly of repair and signaling proteins at sites of DNA double-strand breaks (DSBs). Ring finger protein 8 (RNF8) is an E3 ubiquitin ligase that triggers ubiquitination at the site of DSBs. The present study aimed to identify whether and how RNF8 small interfering RNA (siRNA) treatment enhances the radiosensitivity of irradiated human NPC cell lines. The CNE1, CNE2, and SUNE human NPC cell lines were stably transfected with a constructed RNF8-targeting siRNA expression vector. Western blotting was used to detect the effectiveness of RNF8 downregulation by RNF8 siRNA. The siRNA-transfected (RNF8-) and non-transfected (RNF8+) cells were irradiated at different doses by a linear accelerator. The growth inhibition ratio and apoptosis rate were detected by the methyl thiazolyl tetrazolium (MTT) assay and flow cytometry, respectively. The ataxia-telangiectasia mutated (ATM), DNA-PKcs, Chk1, Chk2, Nbs1 and Ku80 protein levels in each group were determined. The growth inhibition ratio and apoptotic percentage of RNF8- cells were higher than those of the RNF8+ cells in each of the three cell lines. Lower protein expression levels of Chk1, Chk2, ATM, and Nbs1 were observed in the irradiated RNF8- cells compared to the irradiated RNF8+ cells in each of the three cell lines (P<0.01). As a result, a conclusion could be drawn that RNF8 recruits and ubiquitinates many factors to repair DNA damage, including DSBs, thereby conferring radioresistance to NPC cells.