The J-integral is applied to characterize the fracture initiation of phenolphthalein polyether ketone (PEK-C) for which the concepts of linear elastic fracture mechanics (LEFM) are inapplicable at high temperatures for reasonably-sized specimens due to extensive plasticity. The multiple-specimen resistance curve technique recommended by the ASTM is the basic method employed. The values of J(IC) increase with increasing temperature. The crack tip-blunting of PEK-C is observed. The parameters, such as crack opening displacement (COD), delta0, and the stretching increment DELTAa(bl) are introduced to describe the blunting phenomenon. The delta0 increases with increasing temperature, as does DELTAa(bl). This indicates that the blunting occurs easily as the temperature increases; i.e., as the material’s yield stress, sigma(y) steadily falls. The relationships between delta0, DELTAa(bl), and sigma(y) are also discussed.