In order to investigate the electrical properties of transformer oils for large power, the characteristics of AC and Impulse breakdown in gap length of 1.0~2.5mm and that of volume resistivity were researched in temperature range of 20~$100^{circ}C$. An geometrical capacitance of electrode with coaxial cylindrical shape for measuring the volume resistivity was 16pF, and highmegohm meter with model no. VMG-1000 was used, and also the applied voltage were DC 100, 250 and 500V. In the dependance of breakdown characteristics due to electrode gap length, it was confirmed that breakdown voltage was nearly uniform by volume effect according to the increase of gap. In the characteristics for AC breakdown, the dielectric strength was increased to $90^{circ}C$ but decreased over $90^{circ}C$, and also in case of impulse breakdown, it was increased to 7$0^{circ}C$ and at dated $70^{circ}C$ over in temperature range. The calculated mobility of oils in the characteristics for impulse breakdown were about $10^{-5}$~$10^{-4}cm^2/V{cdot}S$, and the value of volume resistivity was almost invariable in low temperature range, regardless of voltage by the stable thermal properties, and it indicated a peak at $50^{circ}C$ and had a sudden change to decrease over that temperature, and also the value of volume resistivity in 250V/mm at $80^{circ}C$ is suitable for the International electrical standards, it was confirmed.