The main effects of ultrasonic sludge are solubilization and release of organic components and biodegradability enhance of sludge by disrupting the physical, chemical and biological properties sludge. The objective of the current study was to determine the effect of ultrasound on anaerobic digestion of oil refinery sludge subjected to different pretreatments. A 300-ml anaerobic reactor was filled with 250 of the waste activated sludge and placed on a hot plate magnet for 41 d. A vibrato bath (40 kHz, 100W) was used in 30 min, 60 min and three cycles of 15 min. Mixed liquor suspended solids (MLSS)was ranged between 28100±282 and 12000±642 mg/L. The data were collected in a log phase condition. TSS, VSS, TCOD and BOD5 were measured following the standard methods. The combined reactor had the best performance in sludge digestion compared to anaerobic reactor with/ without ultrasonic pretreatment. Between two irradiation times (30, 60 min) and the same situation, 60 min ultrasonic pretreatment was found to be more effective. Ultrasonic pretreatment accelerated the digestion process. In this project, oil refinery sludge with industrial feature attained to log phase after 41 days.