The C-alkylation of phenylacetonitrile in a solvent-free liquid-liquid system using phase transfer catalysis under ultrasonic irradiation was investigated. The effects of ultrasonic intensity, inner diameter of the reactor and concentrations of the phase transfer catalyst as well as the potassium hydroxide on the reaction rate were examined. The ultrasonic intensity had an optimum value for the reaction rate. The effect of the ultrasound to promote the reaction was attributed to the extremely large interfacial area between the two liquid phases. The high intensity ultrasound generated a large number of cavitation bubbles in the vicinity of the surface of the ultrasonic horn tip. Those bubbles prevented the acoustic stream induced by ultrasound from propagating to the bottom of the reactor, so that not many emulsion drops existed in the reactor, and the reaction rate was low. The reaction rate using ultrasonic irradiation was much faster than that using mechanical agitation. The reactor with smaller inner diameter produced a faster reaction rate. The reaction rate increased with the concentrations of TBAHS and potassium hydroxide.