In order to investigate the effect of short glass fibre on crack growth, the crack tip was observed using double-cantilever beam samples machined from injection-moulded PMMA plates with a low short glass fibre content by means of optical interferometry. It was shown that the interference fringes were distorted around the fibre crossing the crack planes over a region two to four times larger than the fibre diameter, and that the fibre depressed the crack opening due to bridging between the crack planes, thus shortening the craze zone length in front of the crack tip and reducing the crack growth rate, leaving the arrest line on the fracture surface.