Phenolphthalein poly (ether ketone) (PEK-C) [GRAPHICS] can fail by tearing instability when the elastic contraction is greater than the plastic extension due to crack growth. Tearing instability (TIS) theory developed by Paris and co-workers describes the effect of specimen geometry on the ductile fracture properties of polymers. The stability of crack growth in three-point bend specimens depends on the specimen’s dimensions. In this article, tearing instability theory is applied to describe the ductile tearing instability of PEK-C at different temperatures. The temperature dependence of tearing modulus T(m), dJ/dDELTAa, and the relationship between dJ/dDELTAa and yield stress are discussed.