Poly(trimethylene terephthalate-block-tetramethylene oxide) (PTT-PTMO)/polyhedral oligomeric silsesquioxane (POSS) nanocomposites, synthesized by in situ polymerization, are characterized in detail in terms of interfacial interactions between functional groups on the surface of POSS and polymer chains. The impact of POSS incorporation on the molecular mobility is investigated by means of dielectric spectroscopy. It is found that PTT-PTMO/POSS nanocomposites exhibit higher values of stress at strain of 100% that in comparison with neat PTT-PTMO might result from the influence of POSS particles on the molecular mobility of polymer chains, which is probably due to the interfacial interactions between functional groups on the surface of POSS particles with the polymer matrix. Moreover, it is found that there is no variation of the segmental relaxation for the nanocomposite investigated and there is an increase in the interfacial polarization induced by the presence of POSS as seen in the increment of the real part of the dielectric permittivity.