The fatigue properties of four high strength steels with same strength class but containing different inclusion sizes were investigated using an ultrasonic fatigue testing machine in the gigacycle fatigue regime. The fracture surfaces were observed using field emission scanning electron microscopy (FESEM) and element distributions at the crack origins were measured by an electron probe microanalyzer (EPMA). The fatigue behavior can be divided into three categories:
(a) The S-N curve displays a continuous decline and the internal cracks initiated from the large oxide inclusions for commercial 50CrV4 steel in which the average inclusion size is about 29 lm. (b) Step-wise S-N curves were observed for clean 54SiCrV6 and clean 50CrV4 steels in which the average inclusion sizes are about 3.0 and 2.4 lm respectively. Most fatigue failures originated from the VC inclusion clusters at the lower stress amplitudes. (c) For clean 54SiCr6 steel in which the inclusion size is smaller than 1 lm, the fatigue cracks did not initiated from inclusions or inclusion clusters but from the region enriched with carbon. S-N curve shows that the fatigue failure hardly occurs from 10 6 to 10 9 cycles, in other words, the fatigue reliability can be substantially improved in the super long fatigue life regime.