The electromagnetic properties and thermal behavior of Mn-Zn ferrite cores used for the blocking filter in he Power Line Communication(PLC) application were investigated as the function of additives. The highest density and permeability of 4.98 g/㎤) and 8,221 respectively were obtained to the specimen with composition of MnO 24 mol%, ZnO 25 mol% and Fe$_2$O$_3$ 51 mol% added MnO$_3$ 400 ppm, SiO$_2$ 100 ppm and CaO 200 ppm since the uniform grains were organized and the microstructures were compacted through reduction of pores. The permeability was increased up to 13,904 as temperature of specimen increased to 11$0^{circ}C$, however, it was decreased precipitously under 100 over 11$0^{circ}C$. The exothermic behavior was observed in the frequency range from 1 kHz to 1 MHz that the maximum temperature of specimens became 102$^{circ}C$ at 1 MHz. In the consequence, the Mn-Zn ferrite core developed by this research will maintain the stable electromagnetic properties since the temperature of ferrite core rose to 93$^{circ}C$ in the range of 10 kHz to 450 kHz bandwidth qualified for PLC.