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This paper reports the monitoring system applied for a long-distance 275-kV gas-insulated transmission line (GIL) using the latest fiber-optic technology. The monitoring system consists of two major parts-one monitors the GIL itself, for example, the insulated gas pressure and the surface temperature of the GIL enclosure, fault localization, etc. The other monitors the conditions of the tunnel, where the GIL is installed. To construct a more simple and reliable system, we have developed and applied certain kinds of optical sensors and measuring equipment. This system was verified to be effective through a loading test on an actual GIL and a high voltage test, etc. and has been put into actual operation service since February 1998.