Superparamagnetic iron oxide nanoparticle (SPION)-based diagnostic properties with accompanying therapeutics such as drugs or genes have been explored for improvement of their therapeutic efficacy. Positively charged SPION-loaded polymersomes was prepared to deliver genes to the target sites; this process was concomitantly monitored by magnetic resonance imaging (MRI). The surface characteristics and morphology were respectively measured by dynamic light scattering and transmission electron microscopy. The complex between the polymer and the pDNA was confirmed by a gel retardation assay. The transfection efficiency and cytotoxicity in vitro were tested by treating of the CT-26 colon cancer cell line with luciferase-expressing plasmids/SPION complex. MRI was also used to check the detectability of SPION in vitro and in vivo. A SPION-loaded polymersome carrying genetic materials was delivered and then accumulated in the tumor site of the murine colon cancer xenograft model after intravenous injection, possibly through a passive targeting mechanism. The accumulation was monitored using clinical MRI. This result indicates that the SPION-loaded polymersome can be applied to MR imageguided gene therapy.