Abstract: A new fatigue theory was proposed by introducing the concept of fatigue characteristic length to deal with the weakness of current fatigue evaluation method. The theory is a unified evaluation method to fatigue problems regardless of the nature and size of the defects. It can also give a reasonable explanation of the sizeeffect on fatigue properties, and meet the gap between fatigue limit strength and crack propagation threshold. By considering the fatigue behavior of the material within the characteristic length, it is observed that there are intrinsic relationships among the SN curves, the εN curves and the Paris law. Compared with the experimental results, the evaluation results based on the present theory agrees well with the empirical results of materials not only with or without defect, but with crack fatigue propagation.