In this paper, we have investigated thermal properties and Impurities content of specimens showing by changing the content of carbon black that is semiconductive materials for underground power transmission. Specimens were made of sheet form with the three of existing resins and the nine of specimens for measurement. Impurities content of specimens was measured by ICPAES(Inductively Coupled Plasma Atomic Emission Spectrometer). Heat capacity(ΔH) and melting temperature(Tm) were measured by DSC(Differential Scanning Calorimetry). The ranges of measurement temperature were from $0^{circ}C$ to 20$0^{circ}C$, and heating temperature was 4$^{circ}C$/min. And then thermal diffusivity was measured by LFA 447. The measurement temperature was $25^{circ}C$. Impurities content was highly measured according to increasing the content of carbon black from ICPAES results. And heat capacity and melting temperature from the DSC results were simultaneously decreased according to increasing the content of carbon black, while thermal diffusivity was increased according to increasing the content of carbon black. Because ionic impurities of carbon black containg Fe, Co, Mn, Al, and Zn were rapidly increasing kinetic energy by vibration of ionic impurities through the applied heat energy.