The study investigates the effects of hathia (0.2, 0.4 and 0.6 wt%) along with impurities such as 0.02 wt% SiO2 and 0.04 wt% CaO on the microstructure and magnetic properties of Mn-Zn ferrites. The specimens have been prepared by the conventional mixed oxide method. It has been revealed that the substitution of hafnium into Mn-Zn ferrites enhances the grain growth. Measurements of initial permeability, resistivity and power loss have been related to the microstructure of sintered ferrites in particular to grain size, porosity and grain boundary phases. The present paper gives a summary that optimum properties could be achieved with additive level of 0.4wt% HfO2. (c) 2005 Elsevier B.V. All rights reserved.