Ni 0.65 Zn 0.35 Cu 0.1 Fe 1.9 O 4 nanoparticles fabricated by a polyvinyl alcohol (PVA) sol–gel process have been investigated by infrared spectra (IR), X-ray diffraction (XRD), transmission electron microscope (TEM) and Mössbauer spectroscopy measurements. The particles annealed at and above 250°C are single phase NiZnCu ferrite with spinel structure. Particles annealed at and above 350°C behave ferrimagnetically, while sample annealed at 250°C is simultaneously paramagnetic and ferrimagnetic. In addition, the transition from the paramagnetic to the ferrimagnetic state can be observed in sample annealed at 250°C as the measuring temperature decreases from 260°C to liquid nitrogen temperature. The magnetic properties of NiZnCu ferrite nanoparticles are clearly size-dependent. The saturation magnetization increases with the annealing temperature. The coercivity of NiZnCu ferrite nanoparticles reaches a maximum when the annealing temperature is 550°C.