Abstract: The present paper is aimed at finding a method of predicting the direction of the fatigue crack propagation and fatigue life under general in-plane load. The main points are:we presented a new fracture criterion that the fatigue crack extension takes place in the direction in which σθ is maximum in the equal twin shear stress Line. The extension directions of cracks predicted by this criterion are more approximate to those obtained by exeperiments than by any other fracture criterions.