This study investigates the fiber structure of a newly developed poly-p-pyridylenebenzobisoxazole fiber. The molecular structure resembles that of poly-p-phenylenebenzobisoxazole fiber, which has a rigid-rod backbone structure with a thin-planar shape; thus, a similar preferential radial orientation of the molecular planes normal to the fiber axis is expected. We have focused on the change of fiber structures induced by copolymerizing hydrophilic groups in a rigid-rod backbone of PBO. The preferential orientation of the crystal planes in various positions of the fiber, the molecular orientation along the fiber axis and the crystal size were analyzed by micro-focus X-ray diffraction. The difference in molecular orientation and crystal size in the various positions was related to the formation of the crystal structure induced by radial preferential orientation of the (200) crystal plane sourced from the rigid-rod backbone structure with the thin-planar shape.