This study concerns stress distribution induced by external force in individual poly-p-phenylenebenzobisoxazole (PBO) molecules in fiber. In reality, there are no fibers having an ideal structure (i.e., composed of infinitely long complete crystal elongated parallel to the fiber axis without defects that disconnect stress transfer in the crystal structure). Normally, real fiber structure has some structural incompletion, such as molecular ends, molecular misorientation, and density fluctuation (inhomogeneity) along the fiber axis. They play the role of heterogeneous stress distribution and reduction of fiber modulus in the fiber under tensile deformation. To carry out such analysis, meridional X-ray diffraction peaks of the PBO fiber under stress were measured and discussed. Distribution of the diffraction peak profile (half-height width of the diffraction profile) was especially considered. Change of the molecular orientation induced by external stress to the fiber was also estimated by measuring distribution of equatorial spots along the Debye ring. It was found that the distribution of the meridional diffraction spots became wider in the meridian, while the peak profile along the azimuthal direction became narrower as external stress was added for all three fibers. The degrees of response against stress came in this order: AS (180 GPa) > HM (280 GPa) > HM+ (360 GPa). Hosemann's analysis was adopted to analyze real crystallite size and disorder parameter (g) of crystallites. It indicated that the crystalline size does not vary but the ordering of periodicity in the crystal lattice starts to loosen as applied stress to the fiber is increased. The stress seems to affect only Ideal micro regions in the crystal structure.