C/C composites are a kind of ideal structural materials used at high temperatures. Their high temperature elastic modulus are very important for the thermal stress analysis and composite structure design. The inductance micrometer can be used to determin the deflection accurately in the three-point bending tests, and the tested results can be used to calculate the elastic modulus at room temperature (RT). However, the application of three-point bending method at high temperature is scarcely reported due to the difficulty of the deformation measurement. In this study, the ralative deflection method based on the relative method was utilized to evaluate the elastic modulus at various temperatures. The measured results of elastic modulus of 2D and 3D C/C composites at RT by inductance micrometer and relative deflection method are similar. The elastic modulus of C/C composites at RT ~ 1700℃ were measured by relative deflection method, and the results show that the maxium of the 2D and 3D C/C composites which appear at 1200 ℃ are 82.22 GPa and 66.73 GPa, respectively. The elastic modulus decreased gradually with the increased temperature ranged from 1200 ℃ to 1700 ℃, and the modulus of 2D C/C composites decrease more quickly. These results demonstrate that the C/C composites measurement by relative deflection method for determining high-temperature elastic modulus is accurate and reliable, which could be used to evaluate the modulus of other high-temperature structural materials.