Magnetic nanoparticles (MNPs) were synthesized by a chemical co-precipitation method, which produces monodisperse, nanosize, benign superparamagetic iron oxide nanoparticles (SPIONs) at a reduced synthesis temperature. The crystal structure, morphology, and magnetic characterization were determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometer, respectively. The XRD pattern shows that the presence of the most intense peak corresponds to the (311) crystallographic orientation of the spinel phase of Fe3O4 MNPs. The crystallite size of 4–6 nm was determined from the XRD pattern by using the Scherrer approximation. A TEM image of the MNP showed the mean diameter of 18 ± 10 nm. Magnetization versus temperature (M–T) curve values were used to determine the blocking temperature and particle size by Néel–Arrhenius law and magnetization versus magnetic field (M–H) curve values were used to calculate the size of magnetic particles by the Langevin equation. The particle sizes were found to be 19 and 10 nm. The MNPs proved to be superparamagnetic by M–H characterization at 10 and 300 K. The saturation magnetizations of 57.54 and 8.38 emu/gm were obtained for Fe3O4 MNPs and bovine serum albumin (BSA)-capped Fe3O4 MNPs, respectively. The blocking temperatures of 150 and 130 K were observed for Fe3O4 MNPs and BSA-capped Fe3O4 MNPs, respectively. The SPIONs reported in this study vividly demonstrated their suitability for tagging biomolecules and their potential for rapid immunoassay application.