U3160 carbon fiber reinforced HT-350RTM polyimide composites (U3160/HT-350RTM) are fabricated by resin transform molding (RTM) process. The variation of weight loss rate of U-3160/HT-350RTM composites under different aging temperature and aging time were investigated, and the relationship between aging failure characteristics of polyimide composites and aging time/aging temperature was established. The failure mechanism during the process of thermal oxidation aging was discussed based on microstructure analysis. The results indicated that, at a certain temperature, the change of weight loss rate of the composites follows the variation law of three degree polynomial. The degradation of polyimide resin results in the increase of porosity of the composites, which especially occurrs on the surface of composites due to the oxidation.