Magnetic Cr 3+x Fe 3+ 2 − x Fe 2+ O 4 (0 ⩽ x ⩽ 0.1) porous nanoparticles were prepared by the aqueous coprecipitation method. The resulting magnetic nanoparticles were characterized by using an x-ray diffraction (XRD), field enhanced scanning electron microscope (FESEM), transmission electron microscope (TEM), and vibrating sampling magnetometer (VSM). The nitrogen gas adsorption/ desorption isotherm showed a microporous structure of the obtained magnetic materials. A rod and round shape of Fe 3 O 4 was observed as using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) surfactant, respectively. The Fe 3 O 4 nanoparticles exhibited superparamagnetic properties with easy separation and re-dispersion in solution by using an external magnet. More remarkably, the saturation magnetization (M s ) was enhanced up to 1.2 times for doping Cr 3+ into the Fe 3 O 4 lattice. The effect of surfactants and Cr 3+ doping concentrations on size and the magnetic properties of Fe 3 O 4 nanoparticles are studied.