Catalytic hydrotreating is the most effective process for upgrading heavy gas oil. However, the operating conditions such as high temperatures (633-673 K), high pressures (8.6-8.9 MPa), and consumption of large amounts of catalyst and hydrogen place constraints and limitations on the process. In this investigation the ultrasonic energy is used to treat the heavy gas oil (HGO) without the use of any additives at atmospheric pressure. The lighter gas hydrocarbons given off during the ultrasonic treatment of HGO were identified as methane, ethylene, ethane, and propylene. The basic nitrogen-containing compounds in HGO were more easily cleaved by cavitational energy than that of nonbasic compound. A maximum nitrogen and sulfur conversion of 11% and 7%, respectively, and a 5% reduction in the viscosity were obtained at the optimized sonochemical conditions. A radical chain mechanism is proposed to demonstrate the reactions of hydrocarbons initiated by ultrasound.