Authors investigated the effect of the amount of prestrain at high temperatures on the retained ductility, fracture stress and flow stress curve of killed mild steel. As the result, it was revealed that the retained ductility of steel was fairly affected by both temperatures and amounts of prestrain and generally reduced more conspicuously by prestraining in tension at high temperatures than at room temperature. It was also indicated that the ductility retained after prestraining at 200°C and 500°C exhibited the lowest values, and that in the range of much prestrain, total ductilities obtained after prestraining at these high temperatures became about 60% of those obtained after prestraining at room temperatures. Moreover, it was observed that the fracture stress was fairly reduced by prestraining in tension at the temperature higher than 400°C (cf. Figs. 7 and 8). On the contrary, the conclusions were led to be rather different when compressively prestrained. In this case, it was found that in the range of prestraining up to about 60%, the retained ductility was hardly reduced at any prestraining temperatures and consequently that the total ductility rather increased with larger amount of prestrain, beyond which the retained ductility began to be apparently reduced as after tensile prestraining. As a whole, it appeared that the retained ductility was less reduced after prestraining in compression than in tension. Similar results were obtained regarding the fracture stress in the same range of pre-compression (cf. Figs. 12 and 13).