We report on the study of the structural and magnetic properties of crystalline Fe-doped ZnO nanoparticles with Fe content up to 10% synthesized by a co-precipitation method. The Rietveld analysis indicates that the Fe-doped ZnO nanoparticles are formed in a single phase wurtzite structure. DC magnetization (M) vs. applied magnetic field (H) curves obtained at 5 K show the occurrence of a ferromagnetic behavior. The coercive field and saturation magnetization depend on the Fe content. At room temperature, M vs. H curves show features consistent with a superparamagnetic state of nanoscale system. The temperature dependence of the AC and DC magnetic susceptibilities show features related to the thermal relaxation of the nano-sized particles. From the AC data analysis, a magnetic transition from the superparamagnetic to cluster-glass state is determined. © 2014 AIP Publishing LLC.