A silane coupling agent, gamma-glycidoxypropy ltrimethoxysilane (gamma-GPS), was used to promote the adhesion strength of composite/metal joints fabricated by co-cure bonding process. The thin layer of silane interphase between the epoxy resin of the composite and the steel surface was investigated using SEM. The structure and properties of the interphase are presented. The effect of the interphase on the adhesion strength is discussed in detail. To evaluate the mechanical properties improvement of lap joints by the formation of silane interphase, static and dynamic lap-shear tests on co-cure-bonded double-lap joints were performed. From the viewpoint of durability of adhesive bonding, fatigue damage and failure mechanisms were analyzed and discussed. The silane interphase formation improved both the adhesion strength and fatigue life of co-cure bonded lap joints by chemical bonding introduced at the interface.