Inaccuracy localization and intrinsic radioresistance of solid tumors seriously hindered the clinical implementation of radiotherapy. In this study, we fabricated hyaluronic acid-functionalized gadolinium oxide nanoparticles (HA-Gd 2 O 3 NPs) via one-pot hydrothermal process for effective magnetic resonance (MR) imaging and radiosensitization of tumors. By virtue of HA functionalization, the as-prepared HA-Gd 2 O 3 NPs with a diameter of 105 nm showed favorable dispersibility in water, low cytotoxicity, and excellent biocompatibility and readily entered into the cytoplasm of cancer cells by HA receptor-mediated endocytosis. Importantly, HA-Gd 2 O 3 NPs exhibited high longitudinal relaxivity ( r 1 ) 6.0 mM −1 S −1 as MRI contrast agents and radiosensitization enhancement in a dose-dependent manner. These finds demonstrated that as-synthesized HA-Gd 2 O 3 NPs as bifunctional theranostic agents have great potential in tumors diagnosis and radiotherapy.