The hot torsion test has been adopted as an important means for the determination of a proper temperature for rotary piercing operation in the manufacture of seamless steel tube.However, according to the means heretofore applied in which the workability of steel is judged by the number of twists to failure, a comparison of ductility at high temperatures is qualitatively possible, but it is almost impossible to judge the occurrence of crack in working from that number of twists to failure. Secondary stress caused by torsion test being considered one of the important factors for occurrence of cracks in the making of steel tube, the author has tried to verify the fact by making experimental studies on seven kinds of carbon steel and five kinds of stainless steel so that the result may be applied to the determination of hot workability, and has confirmed the following matters:(1) Secondary stress which occur in torsion tests acts generally as tensile stress on the part of comparatively higher temperatures and as compressive stress on the part of lower temperatures. The temperature at the time of changing from tensile stress to compressive stress, that is, transition temperature varies with the kind of material.(2) The value of secondary tensile stress differs according to testing temperatures. There is a summit on the correlative curve of these two factors, and the shape of this part varies with the kind of steel and its components. In practice, a higher summit and a wider temperature limit can be acquired in case of more easily workable materials.(3) The temperatures where secondary tensile stress and the number of twists to failure respectively show the highest value do not necessarily coincide and defferent according to the kind of material.(4) Secondary tensile stress varies with strain rate when another testing condition is constant, and at the same time, in some kinds of steel, the highest value, which is not clear in the measurement by the number of twists to failure, is shown by this strain rate. They are, therefore, important factors for the study of strain rate and cracks occurred in the process of rolling.(5) Therefore, from the results mentioned in (1)-(4), secondary tensile stress has a close relation with ductility of materials. The secondary tensile stress is possibly applicable to the determination of cracks caused by working, which is difficult for us to judge by the number of twists to failure.