In order to reveal inherent relation between leaf area，canopy radiation and transpiration on economical fruit trees, dynamic changes of transpiration of 15 yearold apricot trees (Prunus armenica L.) and 16 yearold cherry trees (Prunus avium L.) were continuously monitored with sap flow measuring system in Beiliu orchard in Changping, Beijing from May to July,2007. The meteorological data and soil moisture were also measured. Using time character parameter(K) which represents soil, weather and other environmental factors, leaf area(LA) which represents how much energy is absorbed by fruit trees, and reference crop transpiration(T0) which serves as reference, the dual coefficient method was used to establish the model of predicting transpiration (T=K·LA·T0). The correlation analysis between leaf area, net radiation and transpiration was carried out, and K was determined in May, June and July, which is 0.014, 0.09 and 0.06 respectively. By the comparison analysis between the model and linear regression model in that only the radiation was considered, the accuracy was significantly improved. R2 (0.9,0.85,0.75) in May, June and July was twice , the RMSE (2.50,1.92,1.57) and ARD (12.51％,20.76％,19.62%) were half of latter’s. As reference transpiration which got by PriestleyTaylor (PT) was used in the model to calculate transpiration of 177 days on two fruit trees in three months. Then regressing with measured data, the equation that y = 1.01x was got(R2 = 0.92). It showed that the model was very simple to predict transpiration with high precision and easy to use.