The world faces a huge lack of resources. Produced water is the most significant waste stream in the petroleum industry. Ultrasonic technology is a very promising technique to treat produced water in the hope of reusing it instead of wasting it. In this study, experiments were carried out using a bench-scale ultrasonic reactor for the oxidation of organic matter in produced water, which was collected from a natural gas processing field (a real sample). At atmospheric pressure and room temperature, ultrasonic waves (40 kHz) were applied in a batch system for 30, 60, and 90 minutes at pH values of 7.5, 5, and 2.5. The results showed that at optimum operating conditions at steady state case (pH 5, time 90 min) showed that the removal efficiency of COD was 55%. And by applying the coagulation process as a pretreatment by using ferrous sulphate as a coagulant before sonication, then applying ultrasonic at the optimum point, the experiment showed the COD removal efficiency was 70%.