The ultrasonic peening of weld toes in ultra-fine grain steel was applied to enhance the fatigue behavior. The test results show that the ultrasonic treatment remarkably shifts the S-N curve to the right. The FAT( fatigue strength at 106cycles) is increased by 66 %. The fatigue life at Δσ =200 MPa is extended by 58 times. The mechanism of fatigue strength improvement by ultrasonic treatment was studied. It is shown that two mechanisms improve the fatigue strength of welded joint by ultrasonic treatment: one is the decrease of the stress concentration ratio at weld toes,and the other is the building-up of a compressive residual stress at the weld. The ultrasonic peening of weld toes in ultra-fine grain steel was applied to enhance the fatigue behavior. The test results show that the ultrasonic treatment markedably shifts the SN curve to the right. The FAT (fatigue strength at 106cycles) is increased by 66% The fatigue life at Δσ = 200 MPa is extended by 58 times. The mechanism of fatigue strength improvement by ultrasonic treatment was studied. It is shown that two mechanisms improve the fatigue strength of welded joints by ultrasonic treatment: one is the decrease of the stress concentration ratio at weld toes, and the other is the building-up of a compressive residual stress at the weld.