Mullite interlayer of the carbon fiber reinforced Si-C-N matrix composite (C/Si-C-N) was fabricated by liquid precursor impregnation and pyrolysis (PIP). The mixture of aluminium secbutoxide (A1OBu) and tetraethoxysilicate (TEOS) were used as starting materials. The monophase polymer gel used as mullite precursor could be formed from the mixture in the semi-closed air. A mullite interlayer with a thickness about 500nm is obtained at 850°C- 1200°C. The fiber/interlayer bonds are tight. The carbon fibers are not damaged in the fabrication procedure.