Authors have postulated that the occurrence of micro-cleavage cracks is the main reason for the ductility decrease by compressive prestrain. In this paper, the effect of prestrain on ductile fracture initiation behavior of TMCP steel, which is presented by the relationship between stress triaxiality and equivalent plastic strain, was investigated. Test steels were 10, 30% compressively or 10% tensilely prestrained, and round-notched tension tests were performed. It was found that the limit curve of ductile fracture initiation has not been shifted down by 10, 30% compressive and 10% tensile prestrain, which was consistent with the fact that no micro-cleavage cracks occurred through the tests.