本文应用断裂力学的观点,从裂纹扩展的角度论述了尺寸效应的成因,指出决定疲劳极限的不是裂纹的萌生,而是由滑移面分离产生的显微裂纹是否能够达到临界尺寸,从而找出了疲劳极限和门槛值⊿K_(th)之间的本质联系,并提出了两个计算尺寸系数ε的新公式.这些公式与实验结果符合很好。最后还对该公式的使用条件进行了讨论. In this paper, from the point of view of fracture mechanics, the causes of size effect are discussed from the point of view of crack propagation. It is pointed out that the determination of fatigue limit is not the initiation of cracks, but whether the microcracks produced by the slip surface can reach the critical size, The essential relation between the fatigue limit and the threshold ⊿K_ (th) is proposed and two new formulas for calculating the size coefficient ε are proposed. These formulas are in good agreement with the experimental results. Finally, the conditions of use of this formula are discussed.