The magnetic resonance imaging diagnosis and high-efficiency tumor targeting treatment are of notable importance for cancer therapy. In this study, starch-octanoic acid (ST-OA) micelles were successfully prepared to co-encapsulate the anti-tumor drug doxorubicin (DOX) and superparamagnetic iron oxide nanoparticles (SPIONs), taking advantage of the hydrophobic core of the core-shell micellar structure. Size distribution, morphology and in vitro release behaviors of micelles were investigated. In addition, magnetic properties and the anti-tumor theranostic performance were confirmed by conducting the cellular uptake, in vivo imaging and anti-tumor activity experiments. Therefore, co-encapsulation and specific delivery of the chemotherapeutic drug and contrast agent into tumor cells can realize the diagnosis and treatment of malignant tumor at the same time.