Three-dimensional carbon fiber reinforced mullite composites (3D-B C_f/mullite) are fabricated via Al₂O₃ Sol-gel and polysiloxane infiltration-pyrolysis technology. The results show that nanometer γ-Al₂O₃ powders can be prepared through sol-gel using AlCl₃·6H₂O and (CH₂)₆N₄ as starting materials. Mullite can be synthesized by co-pyrolysis reaction between polysiloxane and sol-gel derived γ-Al₂O₃ at 1400℃ under N₂. The density, flexural strength, and fracture toughness of 3D-B C_f/mullite composites are 1.784g/cm₃, 282.5MPa and 17.7MPa·m^1/2, respectively. It is the large quantities of pores in composites that result in low density and flexural strength of 3D-B C_f/mullite composites.