A technique to modify the interface after the formation of the interface is needed either because of some process requirement or because of a change in the required material function over time. In this article we present a novel technique to form an intermetallic compound at the interface between a coating layer and the substrate by depositing a specific metal film on the top of a coating layer. Based on the property of the intermetallic compound, the interface is modified. Experimentally, the diffusion and interfacial reaction of specimens, with a top-film/middle-film/substrate structure were investigated. With proper choice of the top-film metal, the top-film metal diffuses inward without any significant reaction with the middle film and concentrates at the interface between the middle film and the substrate followed by formation of an intermetallic compound. We discuss conditions for obtaining the above phenomena and present a general guide for choosing the top-film metal. Key factors are (1) the segregation tendency of the top-film metal on the middle-film metal, (2) the activation energy of the diffusion of the top-film metal in the middle film and in the substrate, and (3) the formation of an intermetallic compound between the top-film metal and the substrate in the phase diagram. Examples of intermetallic compound formation by diffusion of the top-film metal are given for a Ti film/Nb film/Cu substrate, a Fe film/Nb film/Ti substrate and other systems.