To reinforce the reliability for stable power supply, several maintenance and monitoring systems for the underground power transmission lines have been already applied in main ultra high voltage lines. As one of the monitoring systems, the measuring of exact conductor temperature in loading cables is necessary to smoothly operate the system. Though it is very difficult to directly measure the conductor temperature in the loading cable, conductor temperature is estimated by measuring heat flows radiated from cable surface, cable surface temperatures and load current. In Japan, 275kV aluminium sheathed oil filled (OFAZV) cables are mainly used as the main ultra high voltage underground power transmission lines, while 275kV aluminium sheathed XLPE (CAZV) cable and wire shielded XLPE (WS/CV) cable are recently adopted due to maintenancefree and non-inflammability. But, it is very difficult to estimate the conductor temperature in case that the eddy current losses in cable sheath are relatively large or the heat flows radiated from cable are not uniform such as the trefoil configuration of the aluminium sheathed cables. Therefore, two methods to estimate the conductor temperature in loading cables in arbitrary configurations are proposed, namely by measuring the cable surface temperature and the heat flows radiated from cable, and by measuring the cable surface temperatures and cable load current. The applicability of those methods is verified by using 275kV 1×2, 000mm2 CAZV and 154kV 1×2, 000mm2 WS/CV cables. One of those methods was experimentally applied in acutual underground cable lines, 275kV OFAZV cable for summer in 1989.