We studied the physical properties of Co 3 O 4 -added Ni-Zn ferrite which were sintered at 1050~110 0 ? C for 2 hours. X-ray diffraction showed a spinel structure, and optical microscopy showed grain sizes of 5 to 10 μm . As the sintering temperature was increased from 1050 ? C to 1070 ? C , the initial permeability and magnetic induction increased, and both of the loss factor and the coercive force decreased. The Curie temperatures were about 2 34 245?C with added Co 3 O 4 . The initial permeability was 350 to 420 and maximum magnetic induction density and coercive force 4870G to 4980G and 0.15 Oe to 0.21 Oe, respectively which were similar to those of Ni-Zn ferrite synthesized in the conventional process. The frequency of specimen was in the range of 1MHz to 300MHz. In the plot of initial permeability vs. frequencies, a 180 ? C rotation of the magnetic domain could be perceived in a broad band of microwave before and after the resonance frequency.