The ultrasonic irradiation for crude oil pretreatment to enhance desalting process was investigated. The intensity and irradiation time of ultrasound were studied. The settling time and the temperature of treatment for crude oil were also studied. Micrographs in this paper showed that the sizes of water drops were enlarged by more than 30% after the proper ultrasonic irradiation. The crude oil was pretreated for 5 min at 80 degrees C in the standing-wave field with frequency of 10 kHz and with ultrasonic intensity of 0.38 W cm(-2). After the pretreatment, the dewatering rate and desalting rate were up to 92.6% and 87.9%, respectively in 90 min of settling time. The resulting final salt content of 3.85 mg L-1 is acceptable for refinery. (C) 2008 Elsevier B.V. All rights reserved.