A system of poly(lactide-co-glycolide)-methoxy poly(ethylene glycol) (PLGA-mPEG) nanoparticles is developed to formulate superparamagnetic iron oxides (IOs) for magnetic resonance imaging (MRI). This system improves the imaging effects, increases the half-life of the IOs in circulation, and reduces their side effects. The IO-loaded PLGA-mPEG nanoparticles were prepared by a modified water-in-oil-in-water double-emulsion technique. Their physicochemical and superparamagnetic properties were characterized by various techniques. In vitro IO release kinetics from the nanoparticles and stability of the IO-loaded polymeric nanoparticles were also investigated. In vitro and ex vivo MRI of the IOs formulated in the PLGA-mPEG nanoparticles show that the saturation magnetization and the r2, r2* relaxivities are enhanced, and the contrast effects are improved in comparison with commercial IOs (Resovist). It is proven that the enhanced superparamagnetic properties are caused by the polymeric nanoparticle formulation but not by the polymeric material itself. Moreover, the PLGA-mPEG nanoparticle formulation achieves 36.9 and 35.6 % less cytotoxicity in comparison with the IOs (Resovist) after 48 h incubation at the same 20 and 50 μg mL–1 Fe concentration, respectively. This research implies that formulation of IOs by nanoparticles of PLGA-mPEG copolymer or other biodegradable polymers could be promising for more effective and sustainable MRI with reduced side effects, which, with targeting probes conjugated to the nanoparticle surface, can be further used to promote cellular and molecular MRI.