In this research magnetic nanoparticles of cobalt–zinc ferrite (Co_1-xZn_xFe₂O₄, x=0, 0.2, 0.4, 0.6, 0.8), have been synthesized by the coprecipitation method, using stable ferric, zinc and cobalt salts with sodium hydroxide, at 80 ˚C. Phase formation and the morphology of the resulting nanoparticles were studied by X-ray diffraction and transmission electron microscopy respectively. Mean crystallite size of the nanoparticles was calculated using Scherrer’s formula and the results show that the mean crystallite size decreased from 13 to 8 nm by increasing x values from 0 to 0.8. The superparamagnetic behavior and dependence of blocking temperature T_B on frequency were studied, using AC-susceptibility measurements at the range of 300 Hz to 100 kHz and fitted by Vogel-Fulcher law. According to this law, it was found that there is a strong magnetic interaction between the ferrite nanoparticles.