Nanocomposites (PMSEPE/Q8M 8 H ) were prepared via solution blending of octakis(dimethylsiloxy)octasilsesquoixane (Q8M 8 H ) into poly(dimethylsilyleneethynylenephenyleneethynyle ne) (PMSEPE). PMSEPE/Q8M 8 H nanocomposites were characterized by Fourier transform infrared (FT-IR) spectroscopy, rheological measurement, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and thermal gravimetric analysis (TGA). The experimental results show that the hydrosilylation reaction in PMSEPE/Q8M 8 H nanocomposites occurs slowly exceeding 180 °C. PMSEPE/Q8M 8 H nanocomposites can be cured at temperatures less than 260 °C and the cube structure of Q8M 8 H keeps stable during the curing process. POSS domains are evenly dispersed in the cured nanocomposite. However, serious aggregation of POSS occurs at 15% Q8M 8 H content. The thermal and thermooxidative stabilities of PMSEPE/Q8M 8 H nanocomposites obviously depend on the content of Q8M 8 H . The incorporation of Q8M 8 H can effectively enhance the thermal and thermooxidative stabilities of cured PMSEPE. PMSEPE/Q8M 8 H nanocomposites can be the candidates for applications in high temperature environment.