Abstract: On the basis of numerous experimental data and practical experiences obtained from extensive evaporation investigation of different types of water regions in China,a formula of evaporation coefficient which could be universally used in China is developed and recommended. The formula,simple in form,not only refleets closely the real picture of the mass transfer at air-water interface,but takes care of the mechanism of free and forced convection in a single expression both in form and function. The new formula has been satisfactorily compared with some well-known relevant formulas and evaporation data measured directly from various water bodies.