Crystal structure, morphology and thermal effects of poly(1,4-phenylene-oxy-1,4-phenylene-oxy-1,4-phenylene pyromellitimide) (PMDA-TPE-Q), synthesized from N,N’-dimethyl pyromellitic bisimide and 1,4-bis(4-aminophenoxy) benzene in the melt, have been studied by transmission electron microscopy (TEM), electron diffraction (ED) and wide angle X-ray diffraction (WAXD) measurements. The unit cell of PMDA-TPE-Q is determined by ED and WAXD to be a two-chain orthorhombic lattice with dimensions of a = 0.823 nm, b = 0.558 nm and c = 2.209 nm. The observed crystal morphology from TEM is primarily lamellar with extended chain conformations. The thickness of the lamellar crystals varies with the polymerization conditions. Possible thermal effects on PMDA-TPE-Q have been studied by annealing the crystals at different temperatures. The results indicate that with increasing annealing temperature, the lateral dimensions of the lattice expand, primarily along the a-axis. The c-axis shrinks slightly with increasing the reflection intensities of (0 0 l) planes when l = odd. This is confirmed by using order correlation changes along these directions measured by WAXD experiments. Crystal defects are found in dark field TEM observations.