The tension, Charpy notch impact and fracture toughness tests on structural steels Q235B and Q345 were carried out under different prestrains. Ductile to brittle transition curves are investigated using Charpy impact tests. The results indicate that the prestrain decreases the impact toughness of both base metal (BM) and weld metal (WM); and increases the ductile-brittle transition temperature as well as the risk of fracture. The prestrain is also found to have a more significant effect in a less ductile material. River marking, the characteristic of cleavage fracture, is observed in the Q235B WM specimens 10% prestrain conditions at 0°C. The fracture surfaces of Q345 WM specimens are characterized by dimple-like structures. The depth of these dimples is decreased under prestrain applied before welding. The CTOD-prestrain curves are obtained using Boltzmann relation.