Positively charged superparamagnetic iron oxide nanoparticle (SPION)-loaded polymersome was prepared in order to deliver genes to the target sites, which was monitored by magnetic resonance imaging (MRI), concomitantly. The transfection efficiency in vitro was tested by treating 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. SPION-loaded polymersome, carrying genetic materials, was delivered and then accumulated at the tumor site of the murine colon cancer xenograft model after intravenous injection, possibly through a passive targeting mechanism. Clinical MRI monitored this accumulation. This result indicates that the SPION-loaded polymersomecan be applied to MR image-guided gene therapy.