Ultrasonically assisted extraction of isoflavones from the stem of Pueraria lobata (Willd.) Ohwi has been carried out with an ultrasonic extracting apparatus (20kHz, electrical power input to the transducer in 0-650W). The influence of the electrical power input and extraction time on the extraction yield is investigated in water, n-butanol, and 95% (by volume) and 50% (by volume) ethanol aqueous solution. The experimental results indicate that the yields of total isoflavones are higher in ultrasonically assisted extraction than those obtained from conventional extraction. Moreover, a mathematical model is proposed, by introducing the electrical power input to index the ultrasound intensity, to describe the behavior of ultrasonically assisted extraction. It is found that the model calculations are in good agreement with the experimental data.