Radiation therapy has demonstrated promising effectiveness against several types of cancers. X-ray radiation therapy can be made further effective by utilizing nanoparticles of high-atomic-number (high-Z) materials that act as radiosensitizers. Here, in purpose of maximizing the radiation therapy within tumors, bovine serum albumin capped gadolinium oxide and gold nanoparticles (GdO@BSA-Au NPs) are developed as a bimetallic radiosensitizer. In this study, we incorporate two high-Z-based nanoparticles, Au and Gd, in a single nanoplatform. The radiosensitizing ability of the nanoparticles was assessed with a series of in vitro tests, following evaluation in vivo in a breast cancer murine model. Enhanced tumor suppression is observed in the group that received radiation after administration of GdO@BSA-Au NPs. As a result, cancer therapy efficacy is significantly improved by applying GdO@BSA-Au NPs under X-ray irradiation, as evidenced by studies evaluating cell viability, proliferation, reactive oxygen species production, and in vivo anti-tumor effect.