Chain coherence length of rigid-rod poly(p-phenylene benzobisthiazole) (PBZT) and its derivatives in the solid state was determined from the wide-angle X-ray diffraction patterns of axially disordered crystal. The degree of the PBZT main chain extension was estimated from the coherence lengths and was compared to investigate the effects of side chain, orientation, heat treatment, and polymer solution concentration. Extremely small coherence length obtained from both highly oriented fibers and powder or bulk PBZT homopolymer suggested that a chain conformation deviated from the fully extended conceptual rigid-rod, supporting the ribbon-like conformation, as was previously predicted by molecular dynamic simulation. The deviation was also found to be highly dependent on the processing conditions. Fibers stretched during spinning exhibited much greater chain extension than the isotropic powder, the bulk, and fibers spun without tension. The chain extension was also dependent on the solution concentration prior to the processing. The PBZT produced from solution above the critical concentration exhibited higher chain extension than those from below the critical concentration. However, side chain attachment to the PBZT main chain or post-heat treatments showed a minimal effect on the extension of the PBZT backbone.