C/ SiC composites fabricated by isothermal chemical vapor infiltration were heat treated. Acoustic emission (AE) was introduced to analyze the variations of AE accumulative energy during monotonic tensile test. The microstructures were examined by scanning electronic microscope (SEM). The results indicate that, for the thin interlayer C/SiC, its tensile strength is close to the initial one after heat treatment at 1 500 , but tensile strengths of the compositescan be increased after heat treatment at 1 700 and 1 900. With the increase of heat treatment temperature, tensile stresses increase significantly and modulus decrease gradually. For the thick interlayer C/ SiC, tensile strengths have little changes and tensile stresses increase obviously but modulus decrease gradually after heat treatment at 1 500 and 1 700, while tensile strength and strain begin to decrease but modulus has little change after the same heat treatment at 1 900. Heat treatment remarkably improves the toughness of C/ SiC composites. The fracture work and AE accumula tive energy increase obviously after heat treatment. For the composites with thin interlayer, the fracture mechanism varies from brittle mode to toughness one, and for the composites with thick interlayer, the toughness is further enhanced.