Abstract: Based on the true stress-strain behavior of the material and with the CEGB-R6 safety assessment method, the failure assessment characteristic of the high steel grade oil casing is created with which the fracture toughness K_I vs the ambient temperature impact value A_KV relationship is studied in an statistical way. And the calculation method for toughness target suitability of defective oil casing is obtained with the failure assessment technique. The calculation method is verified by using such processes as artificial crack, FE numerical simulation and internal-pressure explosion full dimensional experiment. The calculation result is categorically compared with the toughness values as specified in API Spec 5CT. Accordingly, the conclusion is made that the API-specified toughness calculation method is based on the design purpose of preventing occurrence of brittle crack, and only the yield strength and the casing wall thickness are taken into account, while factors concerning the casing such O.D. and the defective geometry as developed during the service are out of the consideration, and thus some considerable potential hazard would exist during actual service of the casing.