Abstract: Some recent researches have shown that the fatigue life of metals consists of initiation life, small crack propagation life. The seperating point of large crack and small crack has been disscussed by some anthers, and so has been done the initiation point. However, the definition of seperating points has not been systematically so far.First, this paper presents a definition of small cracks from the point of view for predicting fatigue life of structures. At the upper bound of a small crack, which is also the seperating point of large cracks and sma11 cracks; at the lower bound, which is also the initiation point, a specimen which contains this small crack has the same fatigue strength as the corresponding smooth specimen. This definition is reasonable logically and philosophically. The basis of this definition, this paper gives the "three stage model for predicting total fatique life . The total fatigue life is composed of initiation life, small crack propagation life and large crack propagation life. The seperating points among the three stages are the upper bound and lower bound of the small crack.The seperating points given in this paper are determinable and can be calculated easily. A general method to determine the seperating points is given, and, as an example, the seperate points are determined for a specimen of LY12-CZ aluminium with a hole. Two other examples in this paper show that the prediction of total fatigue life employing this model agrees will with experimental results.