Abstract: Three dimensional braided carbon/carbon composites (3D C/C composites) were fabricated by chemical vapor infiltration. The thermal expansion behavior of these materials with various densities before and after heat treatment was investigated. It is found that PAN-based carbon fibers (CFs) have an obviously negative coefficient of thermal expansion (CTE) after heat treatment at 1 200 ℃. The CTE of the 3D C/C composites is negative from room temperature to 100 ℃ and is proportion to their density. In the temperature range of 100-1 000 ℃, CTE vs. T curves have an almost a constant slope and expansion behavior of the composite is similar to that of pyrocarbon. When the temperature is above 1 000 ℃, a peak is found in the CTE vs. T curves, indicating that expansion of the pyrocarbon is limited by CFs and the thermal expansion behavior of the composites is determined by both CFs and matrix.