5-FU-based chemoradiotherapy (CRT) and oxaliplatin-based CRT are commonly used therapies for advanced colorectal cancer (CRC). However, patients with a high expression of ERCC1 have a worse prognosis than those with a low expression. In this study, we investigated the effect of XPF–ERCC1 blockers on chemotherapy and 5-FU-based CRT and oxaliplatin (OXA)-based CRT in colorectal cancer cell lines. We investigated the half-maximal inhibitory concentration (IC50) of 5-FU, OXA, XPF–ERCC1 blocker, and XPF–ERCC1 blocker, and 5-FU or OXA combined and analyzed the effect of XPF–ERCC1 blocker on 5-FU-based CRT and oxaliplatin-based CRT. Furthermore, the expression of XPF and γ-H2AX in colorectal cells was analyzed. In animal models, we combined the XPF–ERCC1 blocker with 5-FU and OXA to investigate the effects of RC and finally combined the XPF–ERCC1 blocker with 5-FU- and oxaliplatin-based CRT. In the IC50analysis of each compound, the cytotoxicity of the XPF–ERCC1 blocker was lower than that of 5-FU and OXA. In addition, the XPF–ERCC1 blocker combined with 5-FU or OXA enhanced the cytotoxicity of the chemotherapy drugs in colorectal cells. Furthermore, the XPF–ERCC1 blocker also increased the cytotoxicity of 5-FU-based CRT and OXA -based CRT by inhibiting the XPF product DNA locus. In vivo, the XPF–ERCC1 blocker was confirmed to enhance the therapeutic efficacy of 5-FU, OXA, 5-FU-based CRT, and OXA CRT. These findings show that XPF–ERCC1 blockers not only increase the toxicity of chemotherapy drugs but also increase the efficacy of combined chemoradiotherapy. In the future, the XPF–ERCC1 blocker may be used to improve the efficacy of 5-FU- and oxaliplatin-based CRT.