Abstract: Thermal cold fatigue of glass mat reinforced polypropylene has been studied by measuring the flexural property and dynamic mechanical property of the composites and the interfacial shear strength of the single filament specimen after thermal cold cycling treatment in different ranges of temperature. The results show that thermal cold cycle leads to the interfacial failure and reduction in mechanical properties. The interfacial failure due to thermal stress will increase with increasing the range of temperature and the number of cycles. Under the same conditions, the interfacial failure of composites differs for different relaxation of the thermal stress of interfaces.