An on-line monitoring system of electric power line guard against theft by microwave induction is developed and its construction， technical features and application results are briefly described. The whole system consists of monitoring unit， supervision center and the patrolling personnel. By using the system， a series of experiments are made. The results demonstrate that， when there are large moving objects approaching with or climbing on power towers， the monitoring unit， which is installed at power towers 10 meters high， can make induction signal to moving objects and make alarm by speech and then send message to supervision center. After receiving the massage， the center will inform the people who are on patrol， so the patrol person can catch up with scene in good time and the safety of transmitting lines can be protected to a maximum.