The process of decomposing p-nitrophenol (PNP) with power ultrasound requires strict control of acoustic and electric conditions. In this study, the conditions, including acoustic power and acoustic intensity, but not ultrasonic frequency, were controlled strictly at constant levels. The absorbency and the COD concentrations of the samples were measured in order to show the variation of the sample concentration. The results show significant differences in the trend of the solution degradation rate as acoustic power increases after the PNP solution (with a concentration of 114 mg/L and a pH value of 5.4) is irradiated for 60 min with ultrasonic frequencies of 530.8 kHz, 610.6 kHz, 855.0 kHz, and 1130.0 kHz. The degradation rate of the solution increases with time and acoustic power (acoustic intensity). On the other hand, the degradation rate of the solution is distinctly dependent on frequency when the acoustic power and intensity are strictly controlled and maintained at constant levels. The degradation rate of the PNP solution declines with ultrasonic frequencies of 530.8 kHz, 610.6 kHz, 855.0 kHz, and 1130.0 kHz;the COD concentration, on the contrary, increase.