Abstract: On the basis of the experiment, a synthesized evaluation of welding defects to fatigue strength of welding construction is made by fuzzy sets theory.The evaluation takes into account the interrelations among the category, dimension, location and the interaction of defects.Accordingly, a characteristic parameter μ(X)is proposed to evaluate the fatigue strength of welding construction containing defects.Combining this characteristic parameter μ(X)with the quality zone theory, a μ-S-N-P pattern is developed.The problems concerned by engineers such as how to calculate the reliability of welding construction containing defects can be solved when the status of defects is known.The characteristic parameter μ(X)coincides with the proposed primary fatigue source model.