For the application of superparamagnetic iron oxide nanoparticles in biomedical fields fortarget-specific purposes, they should be ultra small in diameter. We developeda simple one-step synthesis of surface modified ultra small superparamagneticiron oxide nanoparticles (USPIONs) with an average particle diameter of1.7 nm in a polar organic solvent. Polyethylene glycol diacid (PEG) surfacemodified USPIONs synthesized in triethylene glycol were nearly monodispersein diameter and highly water-dispersible. The PEG surface modified USPIONswere tested for use as magnetic resonance (MR) contrast agents. They had a lowr2/r1 relaxivity ratioof 3.4 (r1 = 4.46 andr2 = 15.01 mM−1 s−1) and showed cleardose-dependent T1 and T2 map images, indicating that they will be useful as both target-specificT1 and T2 MR contrast agents due to their ultra small size.