Poly(p-phenylene benzobisoxazole) (PBO) was synthesized in the presence of single-walled carbon nanotubes (SWNTs) in poly(phosphoric acid) (PPA) using typical PBO polymerization conditions. PBO and PBO/SWNT composite fibers were prepared by spinning liquid crystalline solutions in PPA. The morphology and properties of PBO/SWNT composite fibers was investigated. The fibril structure was well developed in PBO/SWNT fibers as well as PBO fiber. However, the SWNT rope was not observed in the inner structure of the peeled fibers, implying that SWNT ropes are exfoliated into the individual nanotube in the solution. After tensile fracture the kink bands are observed in the composite fibers similar to PBO fibers. The observation is consistent with the insignificant effect of SWNT on the compressive strength of PBO fiber.