Abstract: Based on an equation of the dynamic response of cup anemometer and the contrast observations between cup anemometer and sonic anemometer, It is shown that u-error caused by the overspeed of cup anemometer is about 1% and data processing error (DP-error) associated with cup anemometer is the most important one because of distinct calculated methods (vector mean and scalar mean). After the reduction for DP-error, the rational wind speed values could be obtained.