Oxidation resistance of SiC satin fabric (25-29vol%) or aluminosilicate plain fabric (25-33vol%)/mullite matrix porous composites was investigated at 1000-1200°C in air for 24h using thermogravimetric analysis. The composites were produced by a polymer impregnation and pyrolysis (PIP) method using a mullite precursor solution of the mixtures of Si(OC₂H₅)₄ and Al(NO₃)₃. The oxidation rate of the sintered SiC fibers with a stoichiometric SiC composition and well-crystallized microstructure was controlled by the diffusion process of gases through dense SiO₂ layer formed on the SiC surface. The activation energy for the oxidation of the SiC fibers was 299kJ/mol. This value was close to the activation energy for the diffusion of oxygen atom in silica glass. The SiC satin fabric/mullite matrix composite heated in an Ar atmosphere showed pseudoductility due to the delamination along the fabric layers. The four point-flexural strength decreased slightly with increasing heat treatment temperature. However, the composite heated in air at 1100-1200°C showed brittle fracture behavior without delamination because of the increased strength of interfacial bond between the oxidized SiC fibers and mullite matrix. On the other hand, the amorphous aluminosilicate fibers/mullite matrix composite showed little weight change during the heat treatment at 1000-1200°C. Crystallization of mullite occurred in the mullite precursor-derived amorphous solid and aluminosilicate fibers at 1100 and 1200°C in air, respectively. However, no microstructural change was observed in the heat-treated composite. The significant pseudoductility and strength of the aluminosilicate plain fabric/mullite matrix composite were maintained after the heat treatment. The fracture energy increased with increasing heat treatment temperature.