Abstract: The dynamic elastic-plastic fracture toughness JId of a low alloy shipbuilding steel has been examined by low blow test using an instrumented Charpy impact machine.A larger error often occurs in measured results by multi-specimen J-R curve method because of the difficulty in determining the blunting line equation.On the basis of considering the effect of loading rate on plastic-flow stress,a new blunting line equation in dynamic loading has been obtained:J=2NK(0.5εF)nΔa.In the process of JId determination by J-R curve method,the result measured using this equation can get a very good agreement with that measured by low blow test.