With the enhancement of the power of engines, pistons are easy to bring crack caused by the alternative mechanical loads. In this paper an estimated formula of the fatigue life is developed by amending the Paris formula on the basis of the study of the fatigue behavior of pistons. It consists of the horny crack life and the surface crack life. As an example, parameters in the formula for one type of piston are determined. The expression of the shape geometry factor and the stress range value are obtained by carrying out the FEA static and dynamic analysis, respectively. The crack parameters are analyzed based on the fatigue theory. The fatigue life of the piston is calculated using these parameters. Finally, the validity of the estimated formula is verified by the experiments.