Abstract: The effects of gas flow rate, temperature, pH value, initial mass concentration of 4-nitrophenol solution, and ultrasonic intensity on the decomposition efficiency of 4-nitrophenol by ultrasonic irradiation and ozone were studied. The results showed that the decomposition efficiency of 4-nitrophenol increased with the increase of gas flow rate, ultrasonic intensity and pH values (pH≤6), and decreased with the increase of initial mass concentration of 4-nitrophenol solution, while both temperature and pH value (pH>6) were relatively not effective. The decomposition reactions of 4-nitrophenol by US/O₃, US and O₃ separately follow pseudo-first-order kinetics with rate constants 1.50×10^-3 s^-1，3.27×10^-5 s^-1 and 6.63×10^-4 s^-1 respectively, and the enhancement factor of 4-nitrophenol decomposition was around 216%. Ultrasound decomposed ozone,providing •OH augmentation,which was the main cause of synergetic effect. The main intermediates during the decomposition included catechol, o-benzoquinone, hydroquinone, p-benzoquinone, phenol, fumaric acid, maleic acid, oxalic acid and formic acid as measured by high-performance liquid chromatograph (HPLC), gas chromatograph coupled with mass spectrophotometer (GC-MS) and ion chromatograph (IC) techniques. The decomposition mechanism of 4-nitrophenol was also discussed.