The effects of molecular weight of atactic polystyrene-b-poly(ethylene-co-butylene) polystyrene [SEBS] triblock copolymers on the morphology, impact strength, and rheological property of syndiotactic polystyrene (sPS)/ethylene-propylene rubber (EPR) blends are examined. From the morphological analysis, it is observed that the size of dispersed EPR phase in sPS/EPR/SEBS blends decreases and the particle size distribution becomes narrower with increasing amounts of SEES in the blends. It is found that the low molecular weight SEES is more effective in increasing the impact strength of sPS/EPR blend than the high molecular weight SEES. The SEM micrographs after impact test show that the sPS/EPR blends compatibilized by the low molecular weight SEES have a good adhesion between the sPS matrix and dispersed EPR particles, whereas the sPS/EPR blends compatibilized by the high molecular weight SEES exhibit a poor adhesion between phases. Although the complex viscosity of low molecular weight SEES is much lower than that of high molecular weight SEES, the complex viscosity of the sPS/EPR blends compatibilized by the low molecular weight SEES is higher than that of the sPS/EPR blends compatibilized by the high molecular weight SEES. It is suggested from these results that the blocks in the low molecular weight SEES penetrate into the corresponding phase more easily than the blocks in the high molecular weight SEES.