Abstract: Testing of the tube and tube sheet fillet weld was realized by using 192Ir source with small focus. The source was sent to the required location through the central cylindrical hole locator by self-made exposure head device. The test was carried out using backward transillumination manner and the image of the fillet weld could be obtained. The test could ensure the quality of tube-tube sheet fillet weld of heat exchanger and the risk of the operators subjected with radiation dose was greatly reduced because the test was carried in outdoors.