Abstract: With in-situ bending various contents of multi-wall carbon nanotubes (MWNTs) were introduced into the solution polymerization system of poly (p-phenylene benzobisoxazole) (PBO)，and the blends were spun into high performance fibers via liquid crystal spinning. Under the observation with a polarized optical microscope，it is found that the pre-diffusion in poly(phosphoric acid) (PPA) instead of direct adding MWNTs will promise better dispersion in PBO. The mechanical and thermal properties of PBO and MWNTs/PBO fibers were tested by a tensile strength tester and thermogravimetric analyzer. The results show that the tensile strength and modulus of the blend fiber containing 2% MWNTs are 4.69 GPa and 128.8 GPa respectively，which are approximately 24% and 23.5% higher than of the pure PBO fibers. The decomposition temperature of the composites is also improved from 668.9℃ to 700.8℃. When the mass fraction increases to 5%，MWNTs aggregate seriously，giving a negative influence on the spinnability of MWNTs/PBO blend and the mechanical properties of the blend fibers.