Abstract: The determination of CTOD(crack tip opening displacement) has significantly affected the safety margin of design as well as the service life in enginnering critical assessment.Single edge notched tension(SENT) specimens is considered appropriate for the determination of CTOD in pipelines.There are several calculation methods against CTOD of SENT specimens while no unified standard has been agreed.In this paper,comparasion between various methods and the double-clip method for API X70 steel through SENT experiment is conducted.Evaluation of influece on critical crack size by different calculation methods with Crackwise 4.0 is performed.Compared with the double clip method,other calculation methods for CTOD deviated significantly, and the accuracy of fracture toughness has a great inlfuence on critical crack size of ECA assessment.