Emulsifier-free emulsion polymerization of methyl methacrylate was carried out in the presence of hydrophilic magnetite nanoparticles using a persulfate initiator. Before the magnetic nanoparticles were placed into the polymerization system, they were dispersed and stabilized electrostatically with perchloric acid in water (magnetic sol). For magnetic nanoparticles in the sol, the hydrodynamic radius and polydispersity index (PDI) measured by dynamic light scattering was 18.8 nm and PDI of 0.282, respectively. The latex sizes also were determined to be 125-223 nm with a PDI of 0.009-0.065. Magnetic measurements by vibrating sample magnetometry confirmed that the magnetite nanoparticles and magnetic poly(methyl methacrylate) nanospheres exhibited superparamagnetism with zero coercivity. The saturation magnetization of the magnetic polymeric nanospheres ranged from 0.2 to 1.19 emu/g depending on the magnetite content. The polymer chain terminations were affected by the addition of magnetite nanoparticles to the polymerization system. In addition, the magnetite that reduced the average molecular weight of the polymers obtained by gel permeation chromatography should be defined as a transfer agent.