In general, most of the butt welded joints in ship hull structures are not removed of its weld reinforcements, so called as welded. Even if the weld joint contains a certain defect, this joint should be rather assessed passable from the design basis when the failure occurs at the weld toe regardless of the included flaws. According to the above mentioned consideration and to the basis of fatigue strength, the authors propose the method to determine the allowable limits of weld defects. Making use of this technique, some trial examinations are conducted as to two types of faults, blow holes and slag inclusions.