To prevent the accident due to transmission line icing， the icing overhead transmission line on-load is analyzed based on mechanics analysis， and a calculation method for icing thickness is discussed. Using the information of the axial tension， two-dimensional obliquities and weather factors such as wind speed and direction， temperature， humidity， an on-line monitoring system of overhead transmission line icing is developed. In this system， the monitoring systems at the terminals of transmission line transmit data to central monitoring site by GSM/GPRS network. By correlative theory， the expert system at the central site can provide the icing information and gives an alarm if it is necessary. This system can prevent the accident due to icing effectively.