A theoretical model for the estimation of fatigue crack length of tensile-shear spot welded specimen is developed which incorporates the natural frequency and mode variation. The model is based on the concept that the propagation of cracks causes a release of strain energy, which is related to the structural modal shape. The effect of the structural mode shape and crack location is also explained. The model, experimental, and finite element results indicate that the existence of cracks cause the reduction of natural frequencies and change of natural modes, and that the mode shape of the structure and crack location will affect the magnitude of the change of these dynamic variables. The predictions of the model are compared with the experimental data and finite element analysis results and agreement is found to be consistent.