Elongational thinning is observed in model polypropylene/polystyrene (PP/PS) blends filled with nanoparticles at nanoparticle loadings ≥3 vol.% and high strain rates, wherein the nanoparticle network that forms at rest is destroyed by the deformation. The elongational thinning is more obvious in blends with hydrophilic silica surfaces than with hydrophobic silica, apparently due to stronger interaction within the hydrophilic silica network and between PS and hydrophilic silica. Moreover, the elongational deformation of droplets is almost unaltered at lower silica loadings but nearly completely inhibited at higher loadings, owing to resisting the deformation by the enough enhanced elastic modulus of the PS matrix filled with high loadings of hydrophilic silica . In contrast, hydrophobic silica significantly boosts the deformation of droplets in elongation presumably because of the reduced interfacial tension due to the preferential distribution of hydrophobic silica at the droplet-matrix interface, relative to the hydrophilic silica which resides mostly in the PS matrix.