We have discovered that the impact strength of polystyrene is increased by the addition of rigid particles several microns in diameter and at concentrations of 0.01 similar to 1 wt%. As examples, we used barium sulfate particles and crosslinked polystyrene beads. From the observation of real-time deformation of polystyrene incorporating the rigid particles, and from finite element analysis, it is clear that the impact improvement mechanism is related to the formation of voids or peeling layers, and successive extension of crazes from the voids or peeling layers.