Poly(p-phenylene benzobisoxazole) (PBO) has excellent thermal stability and mechanical properties because of its rod-like rigid structure. Preparing nanofibers of PBO using ordinary methods is difficult (e.g., electrospinning) because PBO is not soluble in organic solvents. Herein, we report the preparation of PBO nanofibers with an average diameter of approximately 50 nm via crystallization from a dilute solution in sulfuric acid. In addition, a PBO nanofiber mat was prepared that exhibited extremely high thermal stability and high thermal diffusivity, etc.