Two different woven (2D and 3D) carbon/carbon composites (C/C) and a block carbon have been prepared by chemical vapor infiltration (CVI). The effects of the density and porosity of composites, preform architectures and heat treatment on the thermal expansion properties of the C/C composites were investigated. It is revealed that the coefficient of thermal expansion (CTE) of C/C composites is negative below 100 degrees C, and the CTE values are inversely proportion to its porosity. Comparing with 2D C/C composites, 3D C/C composites have a better thermal stability. Heat treatment can increase the thermal stability of composites by changing interfacial thermal stress. The thermal expansion behavior of C/C composites is considered as the result of interaction between fibers and matrix.