A polynorbornene-based mesogen-jacketed liquid crystalline polymer (MJLCP) containing polyhedral oligomeric silsesquioxane (POSS) in the side chain, PNb10POSS, was synthesized through ring-opening metathesis polymerization. The chemical structure of the monomer was confirmed by ¹H/¹³C NMR, high-resolution mass spectrometry, and elemental analysis. Molecular characterizations on the polymer were performed with ¹H NMR, gel permeation chromatography, and thermogravimetric analysis. The phase behavior of this new organic–inorganic hybrid polymer was investigated by differential scanning calorimetry, polarized light microscopy, one-dimensional wide-angle X-ray scattering, synchrotron-radiation small-angle X-ray scattering (SAXS), two-dimensional wide-angle X-ray diffraction, and high-resolution transmission electron microscopy. With the competitive self-assemblies of the two covalently connected building blocks, namely MJLCP and POSS moieties, PNb10POSS shows various phase structures including an angstrom POSS crystal (Cr), a hexagonal columnar (Col_h) phase and the Cr coexisting, and the Col_h phase at different temperatures. The POSS crystal has a tremendous effect on the liquid crystalline (LC) behavior of the MJLCP. The results show that the competition between the crystallization of POSS and the LC formation of the polymer as a whole results in the complex phase behavior of the MJLCP-based nanohybrid. The polymer self-assembles into an organic–inorganic hybrid inclusion complex on the sub-10 nm scale. This work provides a new approach for the design and synthesis of ordered structures constructed by self-assembly on the sub-10 nm scale.