Radiation therapy has shown encouraging treatment efficacy on many types of tumors. However, the side effects from accumulated high dosage during radiation therapy hindered its wide application in some cases. In this study, BSA capped Au nanoparticles (BSA-AuNPs) are employed as radiation sensitizers, aimed at achieving high efficiency for tumor radiation therapy. Our results showed that BSA-AuNPs with a uniform size of ∼18 nm can reduce colony formation, and increase the DNA double-strand breaks (DSBs) at 0.5 and 2 h post-irradiation compared with treatment by radiation therapy alone in a U87 glioblastoma cell line. Cell apoptosis at 48 h is enhanced after radiation treatment with BSA-AuNPs as the sensitizers. An enhanced tumor regression is also observed on the group with BSA-AuNPs sensitized radiation therapy. No obvious toxicity is observed for in vitro and in vivo experiments. BSA-AuNPs show potential in tumor therapy as radiosensitizing agents.