Abstract: Crop coefficient of maize is one of the most parameters needs for crop water requirements.For estimating the evapotranspiration of maize objectively,in this paper,crop coefficient of maize were calculated by using the daily observational data of soil moisture of 1980～2002. The crop coefficient of maize in Liaoning's 5 areas were ascertained and the change of Kc during maize groeing period was analyzed.The Kc as sixth order polynomial functions of time(10 days) were derived.The results showed that the crop coefficient value of maize in Liaoning every area are obvious difference, but the change of their laws are comparatively unanimous.