The boundary of a weld line is not structurally heterogeneous, so the reflection of US waves with the frequency up to 50 MHz is not observed, that allows to obtain high sensitivity to small defects with opening of 0, 4.10^-3 mm. The best results could be achieved by means of supervision of two-dimensional images of defects.

The programme of data processing makes it possible to measure not only the width of the weld with the accuracy of ±0.1 mm., but the sizes of defects. Fast generation ( ~ 25 sec). of weld images at C and B scanning could be obtained as well. High sensitivity to small defects occurs through utilisation of diffraction of spherical ultrasonic waves' effect.

The proposed method of US defects' measurements could be applied to different technical problems of atomic industry and mechanical engineering.