Osteosarcoma is the most common malignant tumor in the skeletal system with high mortality. Phytic acid (PA) is a natural compound extracted from plant seeds, which shows certain antitumor activity and good bone targeting ability. To develop a novel theranostics for magnetic resonance imaging (MRI) and targeting therapy of osteosarcoma, we employed PA to modify manganese dioxide nanoparticles (MnO 2 @PA NPs) for osteosarcoma treatment. The MnO 2 NPs oligomer was formed by PA modification with uniformed size distribution and negative zeta potential. Fourier-transform infrared spectroscopy, X-ray diffraction, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis demonstrated that PA has been successfully modified on MnO 2 NPs, and the structure of MnO 2 @PA NPs is amorphous. In vitro experiments demonstrated that MnO 2 @PA NPs oligomer can be efficiently internalized by tumor cell, and the internalized NPs can react with H 2 O 2 under acid microenvironment to produce Mn 2+ and O 2 . In vivo experiments demonstrated that MnO 2 @PA NPs oligomer can passively accumulate in tumor tissue, and the accumulated NPs can produce Mn 2+ and O 2 for MRI and targeting therapy of osteosarcoma. In conclusion, we prepared a novel bone-targeting nano theranostics for MRI and therapy of osteosarcoma.