A bulk sample of a nano composite consisting of poly(p-phenylene benzobisthiazole) (PBZT) and a thermoplastic matrix polymer was obtained by polymer blending of a matrix polymer of thermoplastic aromatic polyamide and a reinforcing polymer of a copolymer consisting of a precursor of PBZT and a fragment in common with the matrix polymer, using organic solvent, followed by molding. The phase structure of obtained specimens was varied by controlling the molding process conditions. In particular, the mechanical properties, heat resistance, and chemical resistance of the matrix polymer of a bulk specimen which has a three-dimensional network structure of PBZT were improved drastically, even when only a small amount of the reinforcing material was added.