Epoxy-cyanate (EP-CE) ester composites containing the inorganic-organic hybrid polyhedral oligomeric silsesquioxane (POSS) octaisobutyl(T8)-POSS (oib-POSS) were prepared. These EP-CE/POSS composites were characterized by gelation characteristics, transmission election microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravimetric analyses (TGA) and dynamic mechanical analysis (DMTA). The glass transition temperature (Tg) and thermal decomposition temperature (Tdec) of composites increased compared with the neat epoxy-cyanate ester with the increment of POSS content. Additionally, all the EP-CE/POSS composites exhibited higher storage modulus (E’) values (temperature>Tg) than the pure resins. The loss factor peak intensities decreased as the weight percentage of POSS increases. TEM and XRD showed that phase-separated aggregates of POSS were found in matrix, because POSS did not react with the resin.