To improve the mean-life and the reliability of power cable, we have investigated specific heat (Cp) and thermal conductivity of XLPE insulator and semiconducting materials in 154[kV] underground power transmission cable. Specimens were made of sheet form with the seven of specimens for measurement. Specific heat (Cp) and thermal conductivity were measured by DSC (Differential Scanning Calorimetry) and Nano Flash Diffusivity. Specific-heat measurement temperature ranges of XLPE insulator were from $20[^{circ}C];to;90[^{circ}C]$, and the heating rate was $1[^{circ}C/min]$. And the measurement temperatures of thermal conductivity were $25[^{circ}C],;55[^{circ}C];and;90[^{circ}C]$. In case of semiconducting materials, the measurement temperature ranges of specific heat were from $20[^{circ}C];to;60[^{circ}C]$, and the heating rate was $1[^{circ}C/min]$. And the measurement temperatures of thermal conductivity were $25[^{circ}C];and;55[^{circ}]C$. From these experimental results both specific heat and thermal conductivity were increased by heating rate because volume of materials was expanded according to rise in temperature. We could know that a small amount of CNT has a excellent thermal properties.