In this thesis, ultrasonically induced bubble formation in a thermoplastic resin was investigated experimentally and theoretically. A general purpose polystyrene and polyethylene mixed with polyethylene wax which had been saturated by nitrogen in a pressure chamber were excited by ultrasonic vibration to increase the rate of bubble nucleation. The effect of processing parameters, supersaturation pressure and temperature, on the number and size of bubbles was also examined. The ultrasonically induced bubble nucleation was modelled by modifying classical nucleation theory. Although there is some discrepancy between the theoretical predictions and the experimental results, the theoretical investigation explained the effect of ultrasonic disruption on the bubble nucleation mechanism in a gas supersaturated liquid. The possible applications of the ultrasonic nucleation to various polymer processing techniques will be discussed and finally the application of producing the micrecellular foam by ultrasonic disruption will be proposed.