Q345qC steel is widely used in corrugated steel web. In this paper, the fatigue strength of Q345qC steel between 105~109 cycles was investigated using the ultrasonic fatigue testing technique, with a loading frequency of 20 kHz. The fracture surfaces of specimens were observed with scanning electron microscopy (SEM). The experimental results show that the S-N curve of Q345qC steel continues to decrease with the increase of the number of cycles between 105 and 109 cycles and does not generally exhibit a plateau beyond 107 cycles. The observation of fracture surface shows that the mechanism of fatigue failure is similar in both case of ultrasonic fatigue and conventional fatigue in long life regime. Cracks initiate from a mass defect of specimen surface or an interior inclusion.