We investigated the relationship between magnetic properties of MnZn ferrites and Fe2O3 stoichiometry by measuring magnetic properties, electrical resistivity, and density. We used a scanning electron microscope to observe the fractured surface of sintered samples. MnZn ferrites were prepared by adapting the conventional ceramic technique. Toroidal cores were sintered at 1355degC for 4 h with 4% oxygen. The results show that the initial permeability and power losses of MnZn ferrites strongly depend on the content of Fe2O3. And there is an obvious secondary maximum peak in the curve of mui versus T at about 100degC. The initial permeability mui reaches a maximum value and the power loss is lowest over a wide temperature ranging from 20degC to 180degC when the content of Fe2O3 is 52.5 mol%.