Nanoagents of integrating multiple imaging and therapeutic modalities have attracted tremendous attention for biomedical applications. Herein, we synthesize porous hollow FeO as a theranostic agent for MRI and combined photothermal/chemo cancer therapy. The as-prepared porous iron oxide nanoagents allow for T-weighted MR imaging. Interestingly, we demonstrate that the porous structure endows the nanoagents an outstanding photothermal property for cancer cell killing, in comparison with other types of iron oxide nanomaterials. Under the exposure of an NIR laser, the heat produced by porous FeO can accelerate the release of the loaded drug (e.g., DOX) to enhance chemotherapeutic efficacy, promoting the ablation of cancer cells with synergistic photothermal/chemotherapy.