The competitive effects between the concentration of silica nanoparticles with different surface chemistries and the composition ratio of blends on the coalescence behavior of immiscible polypropylene (PP)/polystyrene (PS) blend under slow shear flow was systematically investigated. When PS was used as matrix in PP/PS blends, after shearing at slow shear rate the PP droplet size was refined at low contents of hydrophilic silica whereas it was coarsened at high contents of hydrophilic silica; however, hydrophobic silica always displayed to refine the PP droplet size. When PS was employed as the dispersed phase in PP/PS blends, the addition of hydrophilic silica induced a coarsened morphology in the filled blends with low contents of hydrophilic silica, but the morphology was refined and exhibited “grape clusters” or spherical droplets structure at high contents; hydrophobic silica reduced the size of the PS dispersed phase at low contents while irregular PS dispersed phases were formed at high contents of hydrophobic silica. It was found that the coarsened morphology was promoted as result of the nanoparticle bridging while the coalescence of the dispersed phase was inhibited by the enhanced viscoelasticity induced by nanoparticles, which controlled the morphological evolution of the filled blends. Moreover, the nanoparticle concentration and composition ratio of the blend played a key role in the morphological evolution of the filled blends. Keywords: Nanoparticles, Immiscible Blends, Shear Flow, Morphology Instability