The core loss of conventional MnZn ferrite varies with temperature. Therefore, for use in a trans-former, the main composition must be chosen so that the loss is lowest between 80°C and 100°C. Substitution of Co for Fe in an MnO-ZnO-Fe2O3 ternary system gives a smaller variation in loss with temperature. The gradient of the loss-temperature curve in the lower temperature region becomes steadily gentler upon substitution of CoO up to 0.5 mol%, irrespective of the ZnO concentration. However, excess substitution gives rise to a steep increase in the loss as the temperature decreases. According to analysis using the Globus’ model, both the improvement in the temperature dependence and the abrupt change in core loss in the lower temperature region accompanying Co replacement could result from a change in the magnetocrystalline anisotropy constant K1.