Abstract: It is important to find out that the effect of the interfacial reaction and the properties of carbon/aluminum strong interface bonding brittle interface layer,and then the rainforced fiber damage.In order to get various status of interface reactions, thermal exposure was used.By use of a scanning electron microscope and analysis of the reaction product (Al₄C₃)the status of interfacial bonding was determined.It is shown that the strength of interface bonding and the thickness of the brittle layer mainly affect the crack propagating method in the loading process of composites. When the interface bonding is stiong, the crack propagates across the fiber so that the strength of composites was decreased.When the bonding is weak, the crack propagates along the fiber surface and the fiber is pulled out, so the strength was increased.As a crack source, the brittle interface layer that has a harmful thickness may lead to fracture of composites below the theoretical strength (ROM).Otherwise the brittle layer can be a throughway for quickly crack propagating.When the temperature of thermal exposure exceed 650℃, the stronger interface reaction occurs and the rainforced fiber may be damaged, and therefore the strength of the composite decreased rapidly.