Tensile and Izod impact tests were used to evaluate effect of silicone oil on mechanical properties of high impact polystyrene (HIPS). Two types of HIPS were used for the study. One contains core-shell type particles with average diameter of 0.4 μm (0.4CSL), and the other salami type particles with average diameter of 1.0 μm (1.0SH). Results show that by adding the silicon oil, total fracture energy for 0.4CSL was increased only at the addition of 0.5 wt% under tensile test, but decreased under Izod impact test; while for 1.0SH the addition of silicone oil increased total fracture energy for both tensile and Izod impact tests. The study also included deformation analysis using optical microscopy. In this paper, the relationship between the fracture energy and the size of the whitening zone is discussed.