Abstract: Ultrasonic fatigue testing is an acceleratedtesting method with a frequency far beyond that of conventional fatigueexperiment. The experimental results show that the fatigue strength at 20\,kHz of50# axle steel and 40Cr steel will diminish continuously as faras 10 10 cycles. It is shownthat 50# axle steel and 40Cr steel have no ``fatigue limit'' as in the conventional fatigue curve. So the design of fatiguestrength based on fatigue testing at 10 7 cycles is not adequate. The SEMshows that the fatigue crack propagation initiates at the subsurfaceinclusions in the regime of N>10 7. Testing system of ultrasonicfatigue, operating principle and design of fatigue specimens aredescribed in this paper.