Osteosarcoma is a common type of bone cancers with a high rate of pulmonary recurrence. High-dose radiation therapy is useful for the ablation of unresectable osteosarcoma. However, it may cause severe adverse effects. To address this problem, we developed D-arginine-loaded metal-organic frameworks (MOF) nanoparticles for improving the radiosensitivity of osteosarcoma. D-arginine, a metabolically inert enantiomer of L-arginine, could produce nitric oxide and down-regulate hypoxia-inducible factor-1alpha (HIF-1α) to alleviate tumor hypoxia. In addition, MOF could also generate free radicals to kill the tumor cells. Results demonstrate that D-arginine-loaded nanoparticles enhanced tumor ablation and prevented the lung metastasis in mice upon radiation therapy. Furthermore, the nanoparticles or radiation alone had relatively low toxicity in cells and mice. Therefore, D-arginine-loaded MOF nanoparticles are relatively safe and highly effective in sensitizing osteosarcoma to radiotherapy.