Abstract: Ultrasonic fatigue properties experiments have been performed with base metal and welded joints of 16Mn steel using a home-made ultrasonic fatigue testing system.The objective was to investigate its fatigue behaviors of welded joints in super long life region.The cycling frequency is about 20 kHz and the load ratio is-1.The testing results show that the S-N curves of both base metal and welded joints are not horizontal in super long life region.The specimens still fail over 10⁷, even 10⁹ stress cycles and there is no fatigue limit between 10⁶ and 10⁹ cycles.The fatigue strength of welded joist is much less than that of base metal.It is dangerous to design the super long life welded structures by current fatigue data.It is necessary to carry out comprehensive research of welded joints in super long life region with different material and different joint style.