This investigation presents a simultaneous and convenient approach to produce a high-performance polyimide with a low dielectric constant by introducing the octa-acrylated polyhedral oligomeric silsesquioxane (methacrylated-POSS) into a polyimide matrix to form polyimide semi-interpenetrating polymer network (semi-IPN) nanocomposites. The differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) results indicate that the self-curing of methacrylated-POSS and the imidization of polyamic acid (PAA) occurs simultaneously. The morphology of a semi-IPN structure of polyimide[POSS-PI/POSS nanocomposites with POSS nanoparticles embedded inside the matrix is elucidated. The POSS particles are uniform and are aggregated to a size of approximately 50-60 nm inside the polyimide matrix. The interconnected POSS particles are observed at high POSS content. The structure is highly crosslinked, so the PI/POSS nanocomposites have an enhanced glass transition temperature. The high porosity of the PI/POSS nanocomposites markedly reduces the dielectric constant of PI because of the nanometer-scale porous structure of POSS.