The crystal structure and morphology of a polyimide oligomer synthesized from pyromellitic dianhydride (PMDA) and 4,4’-oxydianiline (ODA) in the melt state have been studied via transmission electron microscopy (TEM), electron diffraction (ED), and wide-angle X-ray diffraction (WAXD). The unit cell of PMDA-ODA was found to crystallize in a two-chain orthorhombic unit cell with dimensions a = 0.836 nm, b = 0.563 nm, and c = 3.303 nm. This unit cell is different from those cells previously reported for PMDA-ODA fibers or oriented films prepared by an imidization process from poly(amic acid) precursors and annealed at elevated temperatures. However, this unit cell compares well to those of aromatic polyimides based on PMDA and similar diamines. This cell can only be observed at relatively low temperatures. Prolonged annealing at elevated temperature (above 300-degrees-C) always leads to a transformation of this low-temperature orthorhombic unit cell I to a high-temperature orthorhombic unit cell II which is close to those previously reported. This irreversible transformation was also confirmed by differential scanning calorimetry (DSC) experiments. The observed crystal morphology is primarily lamellar with extended-chain conformations. The thickness of the lamellar crystal varies with the polymerization conditions. In addition to the normal single lamellar crystal, twined lamellar crystals with an angle of 69-degrees between the two a-axes are most frequently observed. Other twin crystals with angles of 38-degrees and 25-degrees are also found, but less frequently. Possible lattice packing relationships in these twin crystals are discussed. In addition, lamellar crystals with an angle of 90-degrees between the two a-axes are observed, which is indicative of the epitaxy growth on the crystalline planes (100) and (010).