5-aminolevulinic acid (5-ALA) is widely used in clinical practice because it shows tumor-selective accumulation of protoporphyrin IX, a fluorescent substance and a metabolite of 5-ALA. In addition, some studies have reported that 5-ALA increases the radiation sensitivity of glioma and melanoma. In this study, we investigated the radiosensitization effect of 5-ALA on colon cancer. Mice implanted with the human colon cancer cell line HT29 were administered 5-ALA and subsequently X-ray irradiated. These mice were compared with those not administered 5-ALA. Following multiple irradiations, the tumor volume of the 5-ALA group was significantly lower than that of the non-5-ALA group (day 5: p value=0.0489, day 8: p value= 0.0318, day 12: p value=0.0394). Tumor growth was inhibited by multiple irradiations in the 5-ALA group compared with the non-5-ALA group. These results suggest that 5-ALA also exhibits a radiosensitization effect on colon cancer.