Aiming at avoiding the damages from transmission line windage yaw and the drawbacks of the traditional measuring way, a transmission line windage yaw online monitoring system is developed based on wireless RF technology, ARM technology, wireless communication technology, data acquisition technology. It mainly consists of windage yaw monitor, base station, GPRS network and the monitoring center. The topology of system, the structure chart of base station, schematic diagram and software flow chart of windage yaw monitor are given in the paper. Expert software analyzes the monitoring data. The main function of monitoring center expert software is introduced. All-weather monitoring for the important transmission lines is realized. Up to now, the whole system works well.