The influence of BN on the mechanical properties of carbon fibers was investigated. It is found that the Young's modulus of the derived carbon fibers becomes higher with BN amounts increasing through heat treatments at 1600-2350 A degrees C, while the tensile strength increases at 1600 and 2000 A degrees C and decreases at 2200 and 2350 A degrees C with BN amounts increasing. Carbon fibers' modulus and strength are increased by 6.6 and 7.4 % at maximum, respectively. It can be interpreted that fiber modulus is mainly determined by the preferred orientation of graphene planes and the shear modulus between them, while the strength of carbon fibers strength is determined by not only the two factors but also defects. Through X-ray diffraction, Raman spectra, and scanning electron microscope analysis, it is noted that BN catalysis enhances the preferred orientation of graphene planes and makes little change on the shear modulus but generates more defects above 2000 A degrees C for fierce reaction.