Abstract: High frequency power NiZn ferrites with the composition of Ni_0.5-xZn_0.5CoxFe₂O₄ (x=0, 0. 01, 0. 02, 0.03) were prepared by the conventional oxidation process. The temperature dependence of magnetic properties of the samples was synthetically studied in a wide temperature range of -40 to 200℃. The effect of Co³⁺-doping on the power loss at high temperature was also analyzed. It is found that the grains are more homogeneous and refiner with the addition of Co³⁺. In addition, the temperature dependences of the initial permeability and power loss are also greatly improved, and the lowest power loss temperature shifts to a lower temperature with increasing Co³⁺-substitution. The ferrites with the composition of Ni_0.49Zn_0.5Co_0.01Fe₂O₄ have not only rather low high-frequency power loss, but also a nicer temperature stability in the wide temperature range of -40 to 200℃. The relative temperature coefficient of permeability is 3×10^-6℃^-1, the variation range of power loss is from 130 to 140 kW·m^-3 in the temperature range of 100 to 200℃, and the lowest power loss is 130 kW·m^-3 (1 MHz, 10 mT).