Impact strength and dynamic mechanical analysis of polystyrene/polyphenylsilsesquioxane (PS/PPSQ) in situ blend was studied. Toughness, thermal stability, and storage modulus of the brittle PS matrix could be improved by addition of some much more brittle PPSQ particles when the percentage of PPSQ in this blend was not more than 5%. The toughening mechanism about the deformation of the PPSQ particles, which is due to good adhesion between the dispersed PPSQ particles and the continuous PS matrix and some particle-induced crazes or yielding zones, was discussed. PPSQ has the better effect on the modification of PS than its prepolymer with low molecular weight.