The elastic-plastic fracture toughness of a PC/ABS blend has been investigated simultaneously by the conventional J-integral and the crack tip opening displacement (CTOD) methods for specimen thickness varying from 4 to 15 mm. The critical J (J(c)) and the critical CTOD (delta(c)) are-based on crack initiation. Another new method to determine J(c) based on the close relationship between the J-integral and CTOD has also been developed. Those three critical J(c) values, J(c-81), J(c-NEW), and J(c-BS) Obtained from the ASTM E813-81, the new J method, and the equation J(c) = m sigma(y) delta(c) (1-nu(2)) of the CTOD method, are comparable and are independent of the specimen thickness. The constraint factor m evaluated from the linear elastic fracture mechanics (LEFM) approach using the Dugdale model is about 1.5. A rotation factor r(g) of approximately 2 g was obtained which is also independent of the specimen thickness. The crack propagation resistance, J-curve (dJ/da) or delta-curve (d delta/da), decreases with increasing specimen thickness. A close relationship between CTOD and J-integral has been demonstrated in this study.