This study investigated the synthesis of superparamagnetic iron oxide nanoparticles prepared by a co-precipitation method, using OA as surfactant. This was for the purpose of achievement for biocompatibility and thermal stability for use in magnetic hyperthermia for biomedical applications. Fourier transforms infrared spectra and X-ray diffraction showed that the Fe3O4 MNPs were successfully coated by oleic acid. The results showed that the crystallite and average particle size of the Fe3O4 MNPs were dependent on pH, temperature and stirring speed. The saturation magnetization of the MNPs was proportional to the particle size.