Multifunctional nanoparticles that possess targeting, imaging, and therapeutic functions are currently a surge of interests as some of them have demonstrated unprecedented benefits in cancer treatment. Superparamagnetic iron oxide nanoparticles (SPION) may become one of the candidates to fulfill such multifunctions for several reasons. First, surface chemistry for the modification is well known and thus molecules such as targeting ligands and drugs can be attached. Secondly, they have been used as a magnetic resonance (MR) contrast agent due to their superparamagnetic property. Lastly, they are known to be biocompatible and non-toxic. For clinical MRI applications, it is essential to engineer or passivate the labile surface of SPION with favorable functionalities such as: i) anti-biofouling property to render it resistant to protein adsorption in blood and ii) good dispersibility in physiological medium (i.e., high salt and protein concentrations) without formation of any aggregates during reasonably long time period. Here we report 1) the fabrication of a series of antibiofouling polymer-coated SPION that bear near IR dyes and doxorubicin (Dox) as an anticancer drug, and 2) their application to cancer imaging and therapy in vivo. When the SPION was administered to Lewis lung carcinoma tumor allograft mice by intravenous injection, the tumor was unambiguously detected in T2weighted magnetic resonance images as a 68% signal drop as well as in optical fluorescence images within 4 h, indicating a high level of accumulation of the nanomagnets within the tumor site. In addition, at least 2 times higher accumulation of anticancer drug, doxorubicin, was observed at the tumor area with the Dox-loaded SPION compared to free Dox. It is noteworthy that, despite the fact that TCL-SPION does not bear any targeting ligands on its surface, it was highly effective for tumor detection in vivo by dual imaging.