Hemp and jute fibre reinforced polyester composites were fabricated to various fibre volume fractions (V-f) up to 0.45. Laminates reinforced with a chopped strand mat (CSM) glass fibre were also manufactured. The tensile properties of these materials were evaluated. Fracture toughness was assessed, using linear elastic fracture mechanics (LEFM) principles, under quasi-static loading conditions. At equivalent V-f (0.2) it was found that the fracture toughness (K-lc) of the CSM glass fibre reinforced material was approximately 3 times greater than that of the natural fibre reinforced laminates and an order of magnitude greater than the unreinforced polymer alone. Critical strain energy release rates (G(c)) and plastic zone radii were computed. The G(c) of the natural fibre reinforced laminates was approximately an order of magnitude lower than that of the CSM reinforced material at the same V-f. It was hypothesised that the size of the crack-tip plastic zone influences the energy absorbing capacity of the material. By comparing the relative volumes of the plastic zones, implications regarding the toughening mechanisms operative in natural fibre reinforced composites have been made. The applicability of LEFM to characterise toughness in these materials is discussed.