Nanoscale blending of aromatic and aliphatic polyimides has been attempted by employing corresponding poly(amic acid) precursors in order to elucidate clues for achieving a semi-molecular composite film. Pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) were used to make the precursor polymer of aromatic polyimide (PMDA-ODA PI) as a semi-rigid rod-like component, whilst bicyclo[2,2,2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BOCA) and 4,4'-methylenebis(cyclohexylamine) (MCA) were used to prepare the precursor of aliphatic polyimide (BOCA-MCA PI) as a flexible coil-like component. The weight ratio of aromatic to aliphatic polyimides was varied from 100:0: to 0:100 by 10 wt.% gap for monitoring the critical composition upon nanostructure changes. The micro/nanostructure of composite films was characterized by using small-angle x-ray scattering (SAXS) and x-ray diffraction (XRD), while the evolution and thermal property of semi-molecular composites were studied by using FT-IR spectroscopy and dynamic mechanical thermal analysis (DMTA). The result showed that the composite films exhibited a single glass transition behavior, which is ascribed to the molecular level mixing, in the presence of copolyimide fractions.