Metal forming and welding operations can introduce residual stresses into structural components. In this study the effect of prestrain on the J-resistance (J-R) curve of subthickness specimens of HY-100 steel was investigated. Prior to testing, the specimen blanks were subjected to four levels of tensile strain up to 10% prestrain. J-resistance curves were determined for unstrained material using multiple-specimen and single-specimen techniques and for the prestrain material using the single-specimen technique. In all cases a half-thickness, full-width, three-point bend specimen geometry was used. Irregularities in the J-resistance curves were correlated with distinct features on the fracture surfaces. In particular, discontinuities in the J-R curve were correlated with the location of ridges along the crack front. The J-R curves obtained after considering the effect of these discontinuities showed a definite influence of the prestraining: the fracture toughness J_Ic and tearing modulus T decrease with increasing levels of prestrain, with T being more sensitive to the prestrain level than J_Ic.