Abstract: Objective　To investigate the effect and possible mechanism of vascular endothelial growth factor (VEGF) down-regulated by small interfering RNA-mediated RNA interference (SiRNAi) on the radiosensitivity of nasopharyngeal carcinoma (NPC) cell line CNE-2. Methods pVEGF-siRNA (CNE-2 group), pNeg-siRNA (CNE-2/Neg-siRNA group), and CNE-2 cells transfected with VEGF and siRNA (CNE-2/VEGF-siRNA group) were constructed. Cell clonality was calculated by colony-forming unit assay after 6MVX-ray irradiation with 0, 2, 4, 6, 8, 10Gy respectively. The changes of cell cycle phase and apoptosis were determined using flow cytometry. Cyclin D1, Cyclin E, P16 and P53 were respectively measured by reverse transcription polymerase chain reaction (RT-PCR). Results Cell survival rate of CNE-2/VEGF-siRNA group was significantly decreased than those of CNE-2 and CNE-2/Neg-siRNA group after 6MV X-ray irradiation with 0, 2, 4, 6, 8, 10 Gy. All the values of radiobiological parameters including D0, Dq and SF2 in CNE-2/VEGF-siRNA group were respectively lower than those in CNE-2 and CNE-2/Neg-siRNA groups. Cell cycle in CNE-2/VEGF-siRNA group was arrested in the G1/S phase. Both VEGF mRNA and protein expression were significantly decreased in the experimental group compared with controls (P<0.05). The proliferation of treated CNE-2 cells was inhibited in vitro. Among Cyclin D1, Cyclin E, p16 and p53 gene, Cyclin D1 mRNA expression was increased significantly at 6, 12, and 24 h after irradiation. Western blot showed that Cyclin D1 protein expression was increased significantly at 24h after irradiation. Conclusion Down-regulating VEGF expression could enhance the radiosensitivity of NPC cells which mechanism might be arresting NPC cells in G1/S phase through Cyclin D1 signaling pathway.