PBO (poly-p-phenylenebenzobisoxazole) is one of rigid-rod polymers, which originated from stream of the ladder polymers made by US Air Force in 1970s. [1]. PBZT(poly-p-phenylenebenzobisoxazole) was firstly examined, then PBO followed, which was advantageous in performance and cost. As for commercialization as a fiber, Dow Chemical developed a new synthesis route of monomer and polymerization process and Toyobo joined the project in 1991. Their efforts bore fruit in 1994. Toyobo took all patent right from Dow Chemical Co. and produces two types of fibers utilizing pilot plant and will start commercial plant on October 1st, 1998. The potential of tensile modulus of PBO fiber is compared with other super fibers. PBO occupies the top in terms of fiber modulus but fails to reach the crystal modulus. In contrast many other super fibers satisfy the crystal modulus. Therefore, our objective of this study is to understand the mechanical properties in terms of fiber structure.