An experimental setup to excite ultrasonic by modulating electrical arc was applied in the submerged arc welding ( SA W) process of 09MnNiDR steel to study its effect on quality of the welds. Arc-excited ultrasonic energy refines grains of the welds and more acicular ferrite ( AF) appears in the fasion zone. It also enhances impact toughness of the joints at -70 ℃significantly. With the ultrasonic of 50 kHz, the toughness is improved by 47% in the fusion zone and by 82% in the heataffected zone (HAZ). The fractography of welds shows that the fracture is changed from cleavage fracture to gliding fracture while applying ultrasonic vibration.