Effect of silica nanofiller on the deformation response and morphology of low- and high-density polyethylene (HDPE, LDPE) and isotactic polypropylene (PP) modified with fumed silica was investigated. The dynamicmechanical thermal spectroscopy, differential scanning calorimetry, optical microscopy, and density measurements were carried out to determine the temperature dependence of storage and loss moduli as well as nanocomposite morphology. It was demonstrated that the degree of matrix reinforcement is considerably affected by the extent of matrix crystallinity, especially, in the temperature range from (T-m-130 degrees C) to T-m. Based on experimental evidence and literature review, it is proposed that this phenomenon may be attributed to the alpha-mechanical relaxation process occurring above matrix T-g. As a result of adding silica into the melted matrix, mobility of chains in contact with silica particles became reduced. This caused substantial changes in morphology of these semicrystalline nanocomposites.