Abstract: In narrow-bore tube circumferential butt welded joints radiographic test, in accordance with the standard JB/T 4730.2—2005, if multi transilluminations can not be performed due to structural reason, a single transillumination through oval or overlap imaging can be used for the same result. While radiographic testing the narrow-bore tube circumferential butt welded joints, since the thickness of tube that rays penetrate is different, therefore the length and blackness of defect distortion reflected on the film is different. Research about the characteristics of the changes can assess the defects on the film accurately, thus leading to an as accurate as possible assessment of the quality of the weld. According to these characteristics, the transillumination oval and overlay images were divided into four regions to analyze.