The influence of annealing temperature on the fracture behaviour of a commercial extrusion-grade isotactic polypropylene was studied. Fracture mechanics analysis was carried out at room temperature and at a low crosshead speed under J-controlled conditions. Parameters characterizing fracture initiation, J(IC), and crack propagation, T-M, were determined. Some thermal treatments induced "ductile instability" after a certain amount of crack extension while others strongly enhanced the fracture toughness parameters and promoted completely stable behaviour. Aiming to correlate mechanical properties with the supermolecular structure, the different morphologies induced by thermal treatments were studied by differential thermal analysis. In addition, a qualitative fracture surface analysis was carried out by SEM. Craze formation appeared to be the principal plastic deformation mechanism present. The degree of crystallinity and the deg ree of interconnection related to the thermal treatment the sample had undergone, are the main structural factors controlling fracture performance.