Abstract: The test method for mode I delamination toughness of composite layers was the double cantilever beam (DCB) test with end notched flexure (ENF) specimen of unidirectional fiber-reinforced resin composites. In order to obtain the mode I delamination toughness of composite structures with curvature, the DCB test was used on the Arc-ENF specimen. The formulas of compliance and G_IC for Arc-ENF specimen were derived by using beam bending theory and Irwin-Kies formula, and the numerical simulation of the DCB test was finished with ABAQUS. Finally, The formulas of compliance and G_IC are proved to be reasonable and effective by comparing the results of theory formulas, numerical simulation and the DCB test, which have reference value for the test and analysis of mode I delamination toughness of DCB specimens with any curvature.