The effect of hydrophilic silica nanoparticles (SiO2) on the shape stability of crystallizable polybutylene terephthalate (PBT) fibrils in polystyrene (PS) matrix under quiescent and shear conditions was investigated using optical-shear technique, differential scanning calorimetry (DSC) and rheometry. The contributions of the crystallization and viscoelasticity to the improved stability of molten PBT fibrils with different nanosilica contents were discussed based on their rheology data and polarized microphotographs. Upon the addition of only 0.05 wt.% SiO2 nanoparticles, the shape stability of PBT fibrils during quiescent annealing increased noticeably due to the rapid crystallization in these filled PBT droplets. With increasing nanoparticle content, the enhanced viscoelasticity of PBT droplets due to the addition of nanoparticles also began to play a role in improving the shape stability of droplets. The addition of silica nanoparticles was also found to suppress the development of nodular morphology on PBT fibrils under shear flow caused by heterogeneous crystallization. It has been suggested that the loading of a relative high content of silica nanoparticles, the application of a rapid quenching and low rate shear flow are in favor of the shape stability of PBT fibrils under shear flow. (C) 2012 Elsevier Ltd. All rights reserved.