Simultaneous introduction of CaO, Co2O3 and MoO 3 dopants into low loss Manganese-Zinc soft ferrites remarkably improves the power loss characteristic of the ferrite cores. Since the driving frequency of switching power supplies has been raised from several KHz to more than 1 MHz, the power loss of the magnetic devices must be reduced. The eddy current loss of soft ferrite will take a major proportion of iron loss in these high-frequency switching power supplies. An attempt was made to develop low loss Mn-Zn polycrystalline ferrites by increasing resistivity to decrease eddy current loss. Doped Mn-Zn ferrites were prepared by conventional ceramic technique and sintered at the temperature of 1320°C for 1-8 hours respectively in air. The DC resistivity was measured by using four-probe methods on sintered disks whose sides were polished and coated with a thin layer of silver paste as a good contact material. The magnetic permeability at room temperature was measured by using an impedance analyzer (Hewlett-Packard, Model HP4294A) with HP-16047 test fixture. The core loss of Mn0.58Zn 0.37Co0.01Fe2.04O4 ferrite doped with 0.08 wt% MoO3 is 370 mW/c.c at 500 KHz/ 70mT.