Nanocomposites of polydimethylsiloxane (PDMS) modified by fumed silica and three kinds of polyhedral oligomeric silsesquioxanes (POSS) were prepared through open two-roll mill blending. Subsequently, the influences of various POSSs on properties of the nanocomposites were studied comprehensively. The addition of POSS significantly enhances the elongation at break of the nanocomposite but lowers the tensile strength and tensile modulus, which are ascribed to the interruption of silica-silica and silica-PDMS interactions. Owing to the good compatibility of octamethylsilsesquioxane (OMS) and PDMS, OMS is dispersed uniformly in PDMS matrix. However, because of the sublimation of OMS, OMS-containing nanocomposite displays the worst thermal stability. Octaphenylsilsesquioxane (OPS) and heptaphenylvinylsilsesquioxane (VPS) in each nanocomposite show aggregation at some extent, but as the agglomerates of OPS which act as stress concentrators are bigger, OPS-containing nanocomposite reveals poorer mechanical properties. Moreover, grafting of VPS onto PDMS chains makes PDMS chains stiffer, resulting in enhanced glass transition temperature (Tg) and thermal stability.