Abstract: According to the inference method of rough set,the low circle fatigue experiment datas for the 225Cr-1Mo steel were analyzed to find out the important affecting parameters on the fatigue life.Firstly the experiment datas were taken logarithm.Secondly the serious datas were treated by the subtract algorithm.Then the numerical digit of the datas was determined and the decision table was built.Finally the most important parameter,the plastic strain was determined from four parameters such as total strain,plastic strain,elastic strain and stress amplitude.The results proved that the fatigue life was able to be expressed as a function of the plastic stain and the method presented in this paper was effective for determining important parameters in the low circle fatigue experiment.