Some small bars 3-25 mm in diameters consisting of high temperature alloy with coarse grain structure, have to possess high reliable and extra good quality. No extra criterion defects whether in surface or inside of the bars can be permitted of existing.

It is well known that traditional ultrasonic testing method possess dead zone, so that non-destructive testing for small bars is generally by means of eddy current method. But eddy current method is sensitive only to surface or near surface defects of bars. It is very difficult to detect internal defects, especially defects in central site of bars.

In order to detect both surface and inside defects in small bars with coarse grain structure, in this paper, a new ultrasonic testing method immersion creep wave focussed method, with no dead zone has been presented. This new method consisting of a newly- developed special line focused creep wave probe, which contains only one piezocrystal, is able to detect artificial defects of flat bottom hole 0.5 - 0.8 mm in diameter or surface crack 0.1 mm in depth, thus it can be seen that this method has no dead zone and is very sensitive for testing the bars. At the same time, this paper gives a detailed description to the principles, structures, functions and properties of the line focused creep wave probe. The results of plenty of experiments shows that this probe and this new ultrasonic testing technique are very satisfactory for testing the coarse grain structure bars 3-25 mm in diameters.