Abstract: During the process of radiographic testing of small diameter tube’s welding seam, by referring with the standard JB/T 4730.2—2005, an analysis is made with regard to the reason why diagonal exposure has to be applied on small diameter tube. A discussion is also carried out on how much the layout of radiography can effect on the indication of longitudinal flaw. Advices are also given as to the position of image quality indicator. When it comes to the execution of inspection standards as well as enhancing crack resolution, all above-mentioned endeavors are to some extent relevant and meaningful.