Abstract: In this paper, the radiosensitizing effects of gadolinium based nanoparticles to X-ray and carbon ion beams were studied by using gadolinium oxide nanoparticles (GON). Firstly, the particle size of GON was measured by transmission electron microscopy, the hydration radius and Zeta potential of GON were measured by dynamic light scattering (DLS), and the stability of gadolinium nanoparticles in the medium was confirmed by ultraviolet absorption spectrum. It was found that GON with 10.0 μg/mL gadolinium concentration enhanced the hydroxyl radical productivity by 1.13 times in aqueous solution under 30 keV/μm carbon ion beam irradiation. In addition, GON had no obvious toxicity to A549 lung cancer cells and normal MRC-5 lung cells. The uptake of gadolinium in human lung cancer A549 cells enhanced with the increase of co-culture GON concentration, and the uptake of gadolinium was 0.73 pg/cell when the co-culture concentration of GON was 10.0 μg/mL. Most importantly, the radiosensitizing effect of GON on A549 cells irradiated with X-rays and carbon ions was assessed with the clonogenic survival assay. Our experimental results indicated that obviously radiosensitizing effect of GON on A549 cells was observed. The radiation enhancement ratio of GON on A549 cells exposed to X-rays and carbon ions was 15.5% and 10.1% at 10% survival level. Since gadolinium has been widely used for magnetic resonance imaging (MRI), the obtained GON is expected to be an ideal material for the diagnosis and treatment of cancer using X-ray and carbon ions radiotherapy.