Response surface methodology (RSM) based on central composite rotatable design (CCRD) was used to optimize the three important reaction variables-methanol/oil molar ratio (M), NaOH catalyst concentration (C) and reaction time (T) for transesterification of crude cottonseed oil under ultrasonic irradiation. A quadratic polynomial model was obtained to predict the methyl ester yield. 98 wt% of the methyl ester yield could be reached at the deduced optimal conditions: methanol/oil molar raito of 6.2:1, NaOH catalyst concentration of 1 wt% (by the weight of crude cottonseed oil) and reaction time of 8 min. Validation experiments confirmed the validity of the predicted model. Moreover, ultrasonic irradiation was proved to be an efficient, energy saving and economically feasible way to produce biodiesel.