Abstract: The mechanical nature of welded structure fatigue damage is the local stress concentration, and the severity of the stress concentration is depending on the change of stiffness.From the point of stiffness coordination, the causes of stress concentration are analyzed.Based on IIW (International Institute of Welding) standards, the relative fatigue life of butt joint is calculated.Results show the fatigue life increase at least 3.86 times with the smoothened weld.The fatigue life analysis of flat beam with strengthen plate shows that the plate thickness increase 1 times i.e., the local stiffness increase 1 times, the fatigue life decrease 1 times.The two examples of IIW confirm that the stiffness coordination is very important to improve fatigue life.Based on the measured dynamic stress, the fatigue life of welded frame is predicted.Result shows that the stiffness coordination can significantly increase the fatigue life.The fatigue life prediction of welded frame is certificated by the experimentation.