Abstract: The fatigue tests of 400 MPa grade and 500 MPa grade two kinds of ultra-fine grained steels were carried out,and fatigue performances of these two kinds of ultra-fine grained steels,as well as mechanism of fatigue crack initiation were investigated. The results show that fatigue properties for 500 MPa grade steel were better than those of 400 MPa grade steel due to its smaller grain size. Under the conditions of stress ratio of -1,loading way of sine wave,and life cycles of 5×106,the conditional fatigue strength for 500 MPa grade and 400 MPa grade steels were 270 MPa and 220 MPa,respectively. The cracks were initiated from the surfaces of the smooth fatigue specimens which had single-fatigue origin.