This article describes the fabrication of hollow manganese/cobalt oxide nanoparticles (MCO NPs) with a tunable size through a redox reaction and the Kirkendall effect for cancer imaging and drug delivery. MCO-70 NPs (with an average size of 70 nm) can act as glutathione (GSH)-responsive contrast agents for dual T1/T2-weighted magnetic resonance imaging (MRI). The degradation of MCO NPs by GSH led to the enhancement of their T1 and T2 signals by 2.24- and 3.43-fold compared with those of MCO NPs before degradation, respectively. Antitumor agents such as doxorubicin (Dox) could be encapsulated inside the cavity of the hollow MCO NPs (MCO-70-Dox) and be released in the presence of GSH. The MCO-70-Dox NPs showed good tumor growth inhibition effects in vitro and in vivo, and can be promising drug delivery vehicles and MRI contrast agents for tumor diagnosis and reporting drug release.