This paper describes the fatigue crack growth and crack opening behaviors in impact fatigue of two alloy steels; one is a high strength low alloy steel HT-60 and the other is an austenitic stainless steel SUS316. Both of the alloy steels have almost the same ultimate tensile strength of about 600MPa. Major attention was paid to the microscopic crack opening behavior under impact fatigue load measured by means of the fine grating method. The results of the crack growth tests indicated that the crack growth rate in impact fatigue is higher than that in non-impact fatigue for SUS316 alloy steel, but not for HT-60 alloy steel. Such a difference in crack growth behavior between two steels was correlated to the following microscopic crack opening behavior of these steels: In the case of SUS316 alloy steel, both of the effective stress intensity range at a crack tip and the effective crack opening displacement behind the crack tip are larger for the crack propagated under repeated impact load than those for the crack under non-impact load, but in the case of HT-60 alloy steel, the difference in those values measured under impact and non-impact loads is relatively small comparing to the case of SUS316 alloy steel.