Abstract: The effect of crack propagation on the changes of natural frequency was studied through simulation of crack propagation process by dispatching element nodes one by one in a finite element model.Based on the damage mechanics principle,the projection area of crack on cross-section was used to define the damage,and to establish the fatigue life prediction model with stress amplitude and mean stress.The fatigue life prediction by this model showed good agreement with experimental results.The proposed life prediction model can effec- tively predict the fatigue life of spot welded joints.