Abstract: Based on the data of radiation and evaporation from Beijing Solar Radiation Station and Quanting Evaporation Station, the Penman's formula is corrected as where Ho is the radiation balance expressed in evaporation units, Ea is the "drying power" determined from windspeed and the vapour-pressure deficit. Evaporation from open water may be expressed by equation E=E0-F/L where F is heat flux downward from water surface. During the increasing temperature season its sign is plus, and in decreasing temperature season minus. Since we lack the data of water temperature gradiant, F cannot be directly calculated. In this paper it is suggested that the regressive equations of E depending on Eo may be respectively established during increasing or decreasing temperature season. Thus, the monthly values of evaporation in warm season in Beijing may be estimated by following formulas