Double-cantilever beam (DCB) specimens are widely used to measure the mode I critical-energy release rate GIc of glass-fiber reinforced polyester composites (GF/PO composites). It showed GIc varied significantly during crack propagation. GIc of GF/PO composites are higher than that of the pure polyester (PP) resin because of the additional force required to fracture fiber bridging in GF/PO compos-ites. Comparative DCB tests were carried out to demon-strate the occurrence of fiber bridging. GIc varied markedly and were much dispersed, which would induce little varia-tion when initial crack length was 40 mm. The fracture characteristics were analyzed by scanning electron micro-scope.