In high-temperature environments that are beyond the capability of graphite epoxy composites, polyimide matrix-reinforced composites are gaining wide acceptance, particularly in aerospace applications. These systems are capable of withstanding high temperatures up to about 315 °C. Moreover, these composites are lightweight and have high strength. This significantly increases the engine thrust-toweight ratio. The most promising resins from the family of polyimides are the polymerization of monomeric reactants (PMRs) and the bismaleimides (BMIs). These resins are glassy polymers [1], and hence they are susceptable to brittle failure with relatively low resistance to crack propagation (fracture toughness). In order to increase their load-carrying capability and improve their fracture toughness, they are commonly reinforced with graphite fibres in the form of woven fabric. In an attempt to study the fracture behaviour and examine the validity of linear-elastic fracture mechanics (LEFM) in characterizing the fracture toughness of these materials, the present study was undertaken. A polyimide matrix resin (PMR-II-20) with a graphite fabric reinforcement was chosen. The reinforcements is Celion G 30-500 carbon fabric with 3000 filaments per toe, 8 harness satin weave and epoxy sizing. The fibre volume fraction is 0.6. Four-point flexural tests were conducted on unnotched and notched specimens, using an MTS hydraulic system at a crosshead speed of 1.27 mmmin -I. The tests were performed in both translaminar and cross-laminar loading. The specimen geometry and load configuration are shown in Fig. 1. The load versus load-point displacement (LPD) was obtained for both un-notched and notched specimens in both translaminar and crosslaminar loading. A detailed description of the type of tests and results generated have previously been presented [2, 3]. In a specimen loaded in four-point bend, the bending stress at the outer fibre is given by