One of the most challenging issue faced by the oil and gas industry is the formation of crude oil emulsion. The emulsion formation will lead to inconsistency in the production performance, which directly affects the economic growth of the industry. Current, conventional de-emulsification methods such as thermal, demulsifiers, and electrical methods are found ineffective in solving crude oil emulsion problem relativity to expected settling time. Therefore, this research aims to propose ultrasonic wave application method for better separation rate to solve crude oil emulsion problem. The effects of ultrasonic waves are investigated by designed experiment under different operating temperature. The optimum operating conditions are investigated and identified through optimization methods in Design Expert Software. The goals of this research are to achieve maximum water and oil separation and minimize the rag layer thickness. Based on the experiments and results, the best operating condition is at operating temperature of 60 °C at 40 μm of ultrasonic frequency of 20 kHz. These parameters were able to provide 73.3 of water separation rate and 20 volume fraction of oil layer after eight hours of bottle test. © 2020 Published under licence by IOP Publishing Ltd.