Recently, the operating frequency of switching mode power supplies has been increased from 25 kHz to several MHz to enhance their efficiency in a smaller size. Although, it was reported that Mn-Zn ferrites can also be used at 1 MHz, only few papers have discussed the possibility of using Mn-Zn ferrites at the frequency higher than 1 MHz. Our early paper reported that the core loss of low loss grade Mn-Zn ferrites is strongly dependent upon the capacitance of grain boundaries of sintered cores. In frequency region between 1 MHz to 3 MHz, the dielectric loss dominates the core loss, while eddy current loss is the major factor of the core loss in 3 MHz to 10 MHz. It is suggested that decreasing the grain boundary capacitance at middle high frequency or increasing the grain resistivity at high frequency region are the appropriate ways for Mn-Zn ferrites to be used at the frequency between 1 MHz and 10 MHz. This paper investigated the core loss of Mn-Zn ferrites in the frequency range between 1 MHz and 10 MHz in order to improve the properties of the Mn-Zn ferrite cores for their potential applications in this frequency region. The powders containing Fe2O3, MnO and ZnO in a molar ratio of 54.2:37.3:8.5 with the addition of CaO and SiO2 from 150 ppm to 450 ppm were prepared by a conventional ceramic process. Samples were sintered at 1150°C to 1250°C for 2 hrs then annealed at various condition. The results show that the sintering temperature and annealing condition altered the grain boundary impedance. For example, the grain boundary capacitance changes from 13 pF to 7 pF, meanwhile, the dielectric loss factor is lowered from 0.28×10-9 to 0.17×10-9, and consequently the power loss decreases from 253 KW/m3 to 145 KW/m3 at 2.5 MHz.