Abstract: In this paper,a method for measuring acoustic power of plane piston transducer based on calorimetry is presented.When the transducer radiates ultrasound into castor oil,the temperature of castor oil will rise.Using 10-channel parallel thermocouples to measure the average temper-ature rise,energy absorbed by castor oil can be obtained.Acoustic power radiated from the tran-sducer can then be calculated.In this paper,the measurement results from the radiation force method are used as a reference.When the peak-to-peak drive voltage of the transducer is 120 V,average of the results obtained using the proposed method and the radiation force method are 5.09 W and 6.06 W,respectively,with a difference of 16%.The overall uncertainty of the method is 9.3%.The method is simple,economical,practical and accurate.Therefore it is a usefule approach to measure acoustic power of plane piston transducer.