We investigated the influence of ZrO2 on the microstructure and electromagnetic properties of MnZn ferrites by characterizing fracture surface micrographs, magnetic properties, and dc resistivity. Powders of Mn 0.68Zn 0.25Fe 2.07O 4 composition were prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350degC for 4 h in N2/O2 atmosphere with 4% oxygen. The results show that the lattice constant and average grain size increase with ZrO2 concentration, but excessive ZrO 2 concentration will result in exaggerated grain growth and porosity increase. The dc resistivity, activation energy, saturation magnetic flux density, and initial magnetic permeability increase monotonically when the ZrO2 concentration is not more than 0.04 wt% and then decrease with further increase of ZrO2 concentration. On the other hand, the porosity, drift mobility, resonance frequency, and core loss decrease initially and then increase with the increase of ZrO2 concentration.