Abstract: The effects of prestrain on the impact toughness of base metal and weld metal of structural steels (16Mn and Q235B steel) were investigated experimentally by use of the Charpy notch impact test at different testing temperatures.The results of the tests show that:for 16Mn and Q235B steel,the impact toughness of both base metal and weld metal decreases with decreasing temperature and the ductile-brittle transition temperature of the two kinds of material increases a little under prestrain.The range of ductile-brittle transition temperature of the base metal and weld metal of 16Mn steel under 5% prestrain is lower than the room temperature (20℃),at which it can be used safely.The impact toughness of Q235B steel is poorer than that of 16Mn under prestrain condition at room temperature.Prestrain has great effect on the impact toughness of base metal.The fracture toughness of base metal of Q235B under prestrain decreases to one thirds of the toughness under none prestrain.All of the results show that the resistance to fracture of Q235B steel under earthquake condition is rather poor.From the research in this paper,the conclude can be drawn that the prestraining effect should be considered when establish objective toughness standard and carry out more effective safety analysis.