A novel MT300 carbon fabrics/phthalonitrile composite was manufactured through autoclave processing. The effects of postcure temperature on mechanical properties of composites were systematically examined, especially for mechanics performance at 400℃. The results show that the flexural modulus and strengths at room temperature improve significantly with the postcure temperature increased from 315℃ to 330℃. As the postcure temperature raised to 375℃, the RT flexural strength decreases constantly, but the RT flexural modulus remains stable. The flexural properties and compressive strengths at 400°C are obviously enhanced by the higher postcure temperature. Moreover, the RT compressive strengths show a downward trend with increasing postucre temperature, but exhibite a remarkable increase for composites postcured at 350℃. The RT and 400°C interlaminar shear strengths both initially decrease but then increase with the postcure temperature increased from 315℃ to 375℃.