Application of rotor UAV to high voltage transmission line inspection is a newly emerged convenient and effective method a- dopted by domestic and foreign electric power industry. Aerial photography is an important inspection approach for rotor UAV, which can vividly and visually capture the problems appeared in high voltage transmission lines and then guide inspection personnel to take further measures. In view of the special problems of target enhancement existing in image preproeessing of rotor UAV high voltage transmission line inspection, an image ehnancement algorithm based on multidirectional filter （MDF） is proposed. Comparing to conventional image enhancement algorithm, the MDF has features of high robustness, real-time. The MDF, therefore, is very fit for the application of rotor UAV high voltage transmission line inspection.