To check the previously proposed crystal structure of poly(p-phenylene benzobisoxazole) [PBO], we performed an X-ray structure analysis for single crystals of low molecular weight model compounds with the following chemical formulas: [GRAPHICS] Both of these two model compounds show essentially tile same molecular and subcell structures as those of PBO: the molecular chains take an almost perfect planar conformation and are packed together with a relative height between the adjacent chains of about 3 A along the chain axis, although fur the polymer the chains are shifted by the same value but in a disordered mode with respect to the direction of the shift (upward or downward), different fi om the regular packing in model compounds. These structural features are reproduced well with energy calculations. Structural ordering in PBO fibers caused by heat treatment at high temperatures, as clarified by X-ray diffraction measurement, are interpreted on the basis of the energy calculations.