Abstract: The high cycle and ultra-high cycle fatigue properties of Q345qC base metal and circular butt weld joint were studied by using the ultrasonic fatigue testing technique. The experimental results show that during 10⁵-10⁹ cycles, the fatigue strength of welded joint is much less than that of base metal, and both their S-N curves descend continuously. Fracture can still occur on base metal beyond 10⁷ cycles and on welded steel beyond 5×10⁶ cycles. The observation of fracture surface shows cracks mainly initiate from welded toes at fusion area or geometric discontinuity at the surface in welded steel. Furthermore, fatigue failure occurs at the geometric discontinuity area in the high stress regime but occurs at welded toes in the low stress regime.