The oxidation behavior of MoSi 2 Mo 5Si 3 composites at low temperature has been investigated by thermogravimetric analyses, X ray diffraction technique and SEM. The results showed that the oxidation was accelerated with the increase of Mo 5Si 3. "PEST" phenomena was caused in couse of low temperature oxidation when the content of Mo 5Si 3 exceeded 16%(mass fraction). The SiO 2 films were not distributed continuously on the surface of composite due to great amount of MoS 3 whiskers, and oxidation was accelerated.