Poly-p-phenylenebenzobisoxazole (PBO) is one of the rigid-rod polymers, the fiber of which shows high modulus and high strength. For these high mechanical properties to be understood, an analysis of the deformation process occurring in the fiber under stress was inevitable. For this purpose, X-ray diffraction measurements were carried out for the fiber under stress. The diffraction peak broadened as the degree of applied stress was increased. According to a Hosemann analysis, in the PBO fiber only the crystalline order was changed along the fiber axis in the crystal structure under deformation, whereas a change in the crystallite size was not detected. The external stress induced heterogeneous stress distribution to the molecule in the crystal; this distorted the X-ray diffraction peak profile, which followed Hosemann's theory.