Friction Stir Welding (FSW) Technology has many benefits for aluminumalloy joining in the aspects of the improved tensile strengths and fatigue properties compared with conventional riveted joints.Now the Friction Stir Welding Technology has been used for joining stiffeners to skins of the craft.The material is anodized 2024-T62 aluminum alloy.The strength and fatigue properties ofjointsmeet the requirements of the design.However,we found the " gray line" on the X-ray film,which is different from the defect of cavity,which is darker than that of the defect of cavity and without significant boundaries.The position of the "gray line" located on the edge of the pin.The microstructures of cross section with the gray line were observed,but no conventional defect is found,even though,the gray line had little influence on the fatigue and static mechanical testing results,and the position of crack kept away from the "gray line".Through tropism histological analysis,micrnstructure observation,we found that small angle between grain boundaries and incident direction facilitated X-ray to pass through and resulted in the gray line on X-ray film.