Abstract: 3D C/SiC composites with PyC interlayer were fabricated by low pressure chemical vapor infiltration(LPCVI). The different expansion behavior of the composites was qualitatively analyzed before and after different heat treatments. The results show that the expansion behavior of 3D C/SiC composites, which was affected by interfacial thermal stress, can be regarded as the result of interaction of fibers and matrix. The thermal expansion behavior of the composites is changed after heat treatment due to the increase of thermal stability and the change of thermal stress and microstructure of the materials. The coefficient of thermal expansion (CTE) of the composites is decreased by the increase of matrix cracks at low temperatures and changed by the redistribution of thermal stress at high temperatures. However, the matrix cracks sealing temperature was not changed by heat treatment.