A systematic study of the effect of the filler on the immiscible blend properties is presented as a function of the blend composition. As a representative of semicrystalline polymer/amorphous polymer blends, a polypropylene/polystyrene system was chosen. The presence of filler in the blends has induced significant changes in the rheological properties. This behavior is a function not only of filler surface treatment but also of blend composition. Solid state dynamic mechanical analysis is performed and the relaxation behavior is correlated with the morphology of the blends. The increase in tensile strength of the blends filled with surface-treated glass beads is related to the enhancement of the adhesion and polymer/polymer interactions.