The relative change in mode II interlaminar crack propagation performance of graphite/epoxy specimens was determined as a function of isothermal overheating up to 300°C. Various periods up to 30 min were chosen to simulate a fire-extinguishing procedure. End-notched flexure (ENF) specimens of 24-ply unidirectional graphite/epoxy laminates with a starter film, Teflon, introduced at the mid-thickness of the laminate were prepared. The mode II cyclic delamination growth rate and interlaminar shear strength were determined. The mode II strain energy release rate was calculated from load-displacement results. The interlaminar shear strength change in the response of thermal damage was determined according to ASTM D2344 standard. Fracture surfaces were also examined using a SEM in order to identify the microfailure modes and clarify the mechanisms of degradation.