Abstract: According to the elastic fracture mechanics,a parameter of lap stress intensity Kθ was used to describe the fatigue strength of lap-welded joints.Various influential factors of the Kθ have been analysed by finite element method.Therefore an empi rical formula for Kθ was deduced.From fatigue experiments of the lap-welded joints specimens made by two kinds of material(GH150 and 1Cr18Ni9Ti),two △K-N curves were obtained.The fatigue life of various complicated lap-welded joints can be estimated more accurately with the △K-N curves presented in this paper.