Abstract: The thermal expansion properties of unidirectional fiber reinforced composites were studied in this paper. The interface adhesive coefficient C was introduced and the thermal elastic theory was used to derive the eilnatien calculating the revised linear expansion coefficient.The linear expansion coefficients of loth glass and carhon fiher reinforced composites had been measurred by thermal analysis method under different interface conditions and different fiber contents.The calculated values coincided with the measured values and the deviations were only less than 10%.