Morphological survey on new PBO fiber (Zylon(R)) was conducted by X-ray and transmission electron microscopic studies. Crystal size, orientation of the crystal, fibrils, microvoids, and fine structure were discussed. It was found that the molecule in the fiber showed high orientation (more than 0.99 in Hermann＇s orientation function for heat-treated fiber) and relatively small crystal sizes in the longitudinal (160 Angstrom) and the transverse (110 Angstrom) directions. Crystal modulus estimated by extrapolation to perfect orientation on the plot of the fiber modulus as a function of fiber orientation (Northolt＇s method) shows discrepancy from the crystal modulus directly obtained by X-ray scattering. This discrepancy means that the Northolt＇s model is insufficient to describe the Young＇s modulus of PBO fiber. Microvoids elongated to the fiber direction were examined by small-angle X-ray scattering and transmission electron microscopic methods. The diameter of the microvoids was 20 Angstrom to 30 Angstrom and the fiber had a very thin microvoids-free layer (0.2 mu m). Preferential orientation of the a-axis of crystal in the fiber was also confirmed. Summarizing these results, a structure model of the PBO fiber was proposed.