Abstract: According to the field experiments from 2001 to 2004, reference crop evapotranspiration in Beijing were calculated by using Penman-Monteith equation and actual crop evapotranspiration were achieved by water balance equation and adjustment coefficient of soil water. Crop coefficients of aerobic rice in different stages were calculated based on the above results. Variations of crop coefficient were also analyzed. The results indicate that average reference crop evapotranspiration during the growth stage in Beijing is 560.1 mm and the highest daily value in the stage from booting to flowering is 6.8 mm/d with a little variations among years. Average crop coefficient for the whole stage is 1.07. The highest value, 1.49, occurres at the stage from booting to flowering, then the stage from flowering to maturity with the value of 1.20, and the lowest value is at the stage of jointing to booting of 0.87. There is a good relationship among crop coefficient of aerobic rice, the days after emergence, and the cumulative temperature above 0℃.