In the present study, we report the synthesis of Zn-ferrite nanoparticles by a soft chemical coprecipitation method. The magnetic properties of Zn ferrite are sensitive to their average crystallite size ( d) which can be varied by altering the synthesis conditions. The d value was controlled by the pH of the solution which is related to the number of moles of OH- ions (x ). The XRD pattern of as synthesized samples showed single phase spinel structure. The estimated d is in the range of 2 to 12 nm. The magnetization studies showed that the sample with d = 2 nm is paramagnetic whereas the samples with d >; 2 nm exhibited superparamagnetic behavior. The magnetization at 10 kG increases from 2.6 to 21.4 emu/g as d increases from 2 to 12 nm. The temperature dependent studies for the sample with d = 12 nm shows symmetric spectra with single resonance peak of Lorentzian shape. As the temperature decreases an additional peak at lower field side of the main signal appears. The g value and the linewidth (ΔHPP) increase with decrease in temperature indicating the increase in antiferromagnetic (AFM) interactions between the ions in tetrahedral and the octahedral sites.