Poly(pyridobisimidazole), PIPD, is known to be the strongest fiber. X-ray crystal structure analysis was carried out by using the imaging plate, on the basis of the neutron structure analysis (Macromolecules 2002, 35, 3942). Two molecular chains, which are slightly deviated from the planar structure, pass through a monoclinic unit cell with parameters a = 13.33 Angstrom, b = 3.462 Angstrom, c (fiber axis) = 12.16 Angstrom, beta = 105.4degrees, and the space group, P2(1)/a-C-2h.(5) Two molecules with different oriented pyridine groups statistically occupied a crystal site. The intramolecular hydrogen bond is formed between NH and O and the intermolecular hydrogen bonds are formed between OH groups extending outside from the molecule.