It is known that when buckling accident occurs, the buckled part easily separates. In this connection, the deteriorating effects of extremely high compressive prestrain on ductility of mild steel were discussed in the present paper. 30mm thick mild steel plates were compressed uniformaly to a few levels of prestrain up to -35%. After prestraining, 2mm V-notch Charpy specimens and compact tension specimens were machined, and tested for evaluating the criteria of Charpy test and fracture toughness. In order to attain to higher compressive prestrain, mild steel plates were deformed by bending to several levels of prestrain up to about -80%. 2mm V-notch Charpy tests were carried out on these plates The Charpy test results of the materials prestrained by two different ways of plastic deformation were compared, and it was found that there is substantially not much difference between them. The deteriorating effect of compressive prestrain was analysed in terms of prestrain level. Criteria of Charpy test and CT test were found to rise almost linearly against prestrain level. As for the Charpy test criteria, the most sensitive one was _vTr₁₅, and rate of rise of it was 3 to 3.7 deg C for one p.c. of compressive prestrain. The results of CT test were found to be more sensitive, and the rate of rise on the stress of yield point of base metal was 3.5 to 4.1deg C. In order to examine the possibility of cracking by brittle fracture from the unnotched part in the case of buckling accident, steel plates were bent to various angle and unloaded. When absolute values of compressive prestrain exceeded the level of ductility of the plate, brittle cracks initiated at the compressive side in the course of unloading. From the fractographic examination, it was found that, at the starting point, fracture surface was covered with not only quasi-cleavage facets but facets of slip planes. After propagation of 2.3mm, fracture surface consisted of cleavage facets.