Tensile properties of two high-impact polystyrenes (HIPS), named 0.84S and 0.45S, were evaluated at four strain rates, from 2.7 x 10(-3) to 1.8 x 10 s(-1), using un-notched dumbbell specimens. Under the strain rates up to 1.6 x 10(-1) s-1, 0.84S showed slightly higher fracture energy than 0.45S, where the fracture energy is defined as the total area under the force-displacement curve, However, when the strain rate was further increased to 1.8 x 10 s(-1), the difference was remarkably increased, due to a dramatic increase of the fracture energy for 0.84S at the highest strain rate, 1.8 x 10 s(-1). The high fracture energy for 0.84S was found to come mainly from the fracture strain. The microscopic examination, using optical microscopy and transmission electron microscopy (TEM), shows the trend of deformation behavior, which supports the tensile test results, and suggests that the adiabatic deformation process, which has long been known to cause the fracture energy increase at high strain rates for many polymers, does not provide a satisfactory explanation for the results presented here. Instead, it is believed that at the highest strain rate the molecular stretch, instead of the molecular flow, dominated the craze fibril deformation and resulted in the craze proliferation around the rubber particle. Such a craze proliferation mechanism did not occur in 0.45S, because unlike 0.84S, the displacement misfit between particle and matrix was insufficient to initiate crazes. As a result, the fracture strain and the fracture energy were limited. We conclude from the study that the different capability to initiate crazes is a dominant factor for the significantly different fracture energy of the two HIPS at the highest strain rate.