The surface morphology of a system of polystyrene incorporating multilayer impact-modifier particles revealed crescent-shaped traces that could be explained as the intersection of the propagating crack and the craze zones initiated by the impact-modifier particles. Th is view is supported by a computer simulation which showed the variation of the shape of the intersection loci with the craze velocity/crack velocity ratio. Hence, by shape comparison or direct dimensional measurement of the crack-craze intersection, the craze velocity/crack velocity ratio at any point of the fracture surface can be determined. Together with data obtained from an instrumented Charpy impact apparatus, the craze velocity and crack velocity can be calculated. These results revealed a 100-fold increase in the crack velocity over a very short distance (similar to 20 mu m) during ductile-brittle transition. This method of interpreting a nd using fracture-surface morphology could be a very useful tool in studying the impact-modification phenomenon of the current system or other systems showing similar fracture-surface morphology.