Fe3O4@C nanocapsules were synthesized via a sacrificial-template method by coating SiO2 nanospheres with an Fe3O4@C double-shell structure, followed by dissolving SiO2 nanospheres through ammonia water under hydrothermal conditions. The nanocapsules show a loading capacity as high as 1300 mg g−1 for doxorubicin (DOX), and the DOX loaded on the surface of the carbon shells displays pH-sensitive release behavior. Drug release experiments were carried out at pH 7.4, 6.2 and 5.0. It was found that the drug release rate at pH 6.2 was about two times as fast as that at pH 7.4, and even faster at pH 5.0. A MTT assay was used to test the cytotoxicity of the DOX, nanocapsules and DOX–nanocapsules, which indicated the low cytotoxicity of the nanocapsules towards cells. The DOX–nanocapsules can be taken up by cancer cells through endocytosis, releasing DOX into the cytoplasm, which was observed by both transmission electron microscopy and confocal microscopy. In vitro magnetic resonance imaging (MRI) experiments validated the potential use of these nanocapsules as MRI contrast agents.