The morphology of solution-cast thin films of wholly aromatic thermoplastic polyimides (TPIs) with various molecular weights (M(w) = 3490-85 800) was studied using transmission electron microscopy (TEM). Thin TPI films suitable for TEM studies were prepared from 0.3% solution in p-chlorophenol/phenol cast onto hot polyphosphoric acid, where the solvent was evaporated. A TPI oligomer of M(w) = 3490-6500 forms droplet-like structure (D structure) during casting, whereas TPIs with higher molecular weights (M(w) = 23 900-85 800) form ellipsoidal and elongated structures. On heating the as-cast film of TPI with M(w)=3490, the D structure is converted into a flat, layer structure with extended chains, which gives an electron diffraction pattern of the single-crystal type. On the other hand, long curved crystals with folded chains develop in the D structure of a TPI with molecular weight of 6500. The supermolecular structures of TPIs with higher molecular weights (M(w) = 23 900-85 800) are noticeably affected by the molecular orientation in the original structure of the as-cast films. Oriented crystallization occurs in the elongated structure, while lamellar crystals develop in the D structure or ellipsoidal structures.