Abstract: Welded joints of austenitic steels are tested by ultrasonic sound caused to be incident on the joint at an oblique angle thereto. Information regarding the size and nature of reflectors in the joint can be derived from the amplitude of the echoes generated at the reflectors in response to the sound beam. To ensure a definite relationship between the amplitude of the echoes and the nature and size of the reflector, the frequency of the sound use for testing is below and upper frequency limit at which the wavelength of sound in the base material equals the largest peak-to-valley distance of the base metal weld interface in any of the consecutive areas of the interface on which the sound beam is incident at any time.