Abstract: Micro mechanical theoretical and FE models have been developed to predict the Coefficient of Thermal Expansion (CTE) of unidirectional composites. The effects of matrix micro cracks and fiber/matrix interface debonding on CTE of composites were considered in the models. These two damages are assumed to affect CTE of composite via causing matrix stiffness degradation and introducing additional strain in interface debonding region, respectively. The model was adopted to predict the CTEs of glass fiber/resin matrix unidirectional composites with various fiber volume fractions and damage intensities. Meso FE models were built to validate the theoretical ones. The results reveal that the matrix damage could reduce CTE of unidirectional composites in longitude, however promote CTE in transverse. Fiber/matrix interface debonding could also bring notable increment of transverse CTE.