In order to clarify the fatigue strength charactaristics of low alloy steels which were strengthed by tempering at low temperature (150∼200°C) after conventional quenching treatment and/or carburizing in generated gas, rotating bending fatigue tests were conducted in laboratory air and dry air using smoothened specimens with 8 mm diameter. The influence of moisture in air was only recognized for the carburized steel. The S-N diagrams of both steels revealed two knees on those curves regardless of the test enviroments and no fatigue limits were observed in the long life range up to 10⁹ cycles (10 months). The reasons were discovered from the results of fractography and the equation proposed by Murakami et al. for the prediction of fatigue limit of steel with micro defects.