Abstract: The uncertainty evaluation of fracture toughness for metallic materials is affected by many different factors,and people's knowledge about the influencing factors is different,which makes the uncertainty evaluation of fracture toughness still at an primary stage.Take Q500 steel for example,the uncertainty evaluation of critical CTOD value for fracture toughness was discussed and calculated in terms of specimen size,testing machine,testers and COD gauge in this paper.The results showed that the measurement error of V_p was the main factor that influenced the uncertainty evaluation of critical CTOD value.