Abstract: The effect of liquid to material ratio, microwave processing time and microwave power on extraction rate of stevioside in leaves of Stevia rebaudiana Bertoni were studied by single-factor experiments.On the basis of single-factor experiments, a quadratic polynomial regression equation of the forecasting model was set up using Box-Behnken design of three factors and three levels.The optimal conditions of microwave-assisted extraction were concluded as follows:liquid to material ratio 22:1 (mL:g) , microwave treatment time 12min, microwave power 500W.The actual detection value of stevioside extraction rate was 90.6mg/g, which was almost equal to the predictable value.