The ice storm of 2008 in south-central China caused much damage to overhead transmission lines, the electrical installations and railway contact conductors. An expert system of ice prevention on overhead transmission lines is developed to predict the ice type (rime or glaze) and anti-icing current intensity based on the meteorological parameters such as air temperature, liquid water content, mean volume diameter of droplets and wind speed which can be obtained by respective sensors. The freezing fraction is used to decide the ice type and anti-icing current intensity is defined based on heat balance equation of conductor surface. Under labview platform, data acquisition board is used to gather physical signals from various sensors that are transmitted to computers and processed by the decision-making model. This expert system can provide reliable suggestions for electrical engineers to prevent icing on transmission lines.