Cubic structured nickel-zinc ferrite nanoparticles (Ni_0.25Zn_0.75Fe₂O₄) have been synthesized by thermal treatment method. In this procedure, an aqueous solution containing metal nitrates as precursors, polyvinyl pyrrolidone as a capping agent, and deionized water as a solvent were thoroughly stirred, dried at 353 K for 24 h, and crushed into powder before calcination to remove organic matters and crystallize the particles. The structure and particle size were characterized by X-ray powder diffraction and transmission electron microscopy. The average particle size increased from 7 to 25 nm with increase of calcination temperature from 723 to 873 K respectively. The magnetic properties were determined by vibrating sample magnetometer and electron paramagnetic resonance electron paramagnetic resonance at room temperature. By increasing the calcinations temperatures from 723 to 873 K it showed an increase of the magnetization saturation from 11 to 26 emu/g and the g-factor from 2.0670 to 2.1220. The Fourier transform infrared spectroscopy was used to confirm the presence of metal oxide bands at all temperatures and the removal of organic matters at 873 K.