Abstract: A elastic-plastic transform formula between crack mouth opening displacement and loading-line crack opening displacement(COD) for straight-notched compact tension(CT) specimens was deduced on the basis of the rigid rotation hypothesis and the work equivalence principle.Cr2Ni2MoV steel was employed to verify the validity of this formula experimentally.As a result of elastic-plastic finite element analysis,the rigid rotation assumption is completely proved.The rotation center of a straight-notched CT specimen is near the crack tip,and has little effect on the material constitutive relationship;consequently,the rotation radius R of the specimen is only related to the ratio of crack length a to specimen width W.The finite element analyses and experiments indicate that more accurate results of plastic work for straight-notched CT specimens may be obtained if using the elastic-plastic COD transform formula.