Chemoradiation therapy (CRT), a combination of X-ray irradiation and anticancer agents as a radiosensitizer, has been found to be an effective treatment for esophageal cancer and has been linked to p53 genetics. The p53 gene family regulates cell-cycle arrest, apoptosis and DNA damage repair. A recently identified ribonucleotide reductase, p53R2, is directly regulated by p53 in the supply of nucleotides for repairing damaged DNA. In the present study, we investigated the improvement in radiosensitivity of human esophageal squamous cell carcinoma (ESCC) cell lines using p53R2 small interfering RNA (siRNA). p53R2 expression in ESCC cells (TE-8) with or without transfection of p53R2 siRNA was examined by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR). The radiosensitivity of TE-8 cells was also measured by cell survival assay. In addition, we investigated the relationship between the expression of p53R2 mRNA in the biopsy specimens of untreated primary tumors and the efficacy of CRT, using RT-PCR. The expression of p53R2 was amplified after X-ray irradiation (14 Gy) and diminished after X-ray irradiation following the transfection of p53R2 siRNA in TE-8 cells. The radiosensitivity of the TE-8 cells significantly improved following the transfection of p53R2 siRNA. In the clinical study, a significantly lower p53R2 mRNA expression was detected in the effective response cases. We demonstrated that p53R2 is associated with the radiosensitivity of ESCC cell lines, and that p53R2 expression is reduced after X-ray irradiation following the transfection of p53R2 siRNA. This protocol could potentially improve the efficacy of radiation therapy.