Interfacial reinforcement with urethane binder (UB) was investigated in a couple of thermoplastic composites, glass fiber / polycarbonate (GF / PC) and glass fiber / polypropylene (GF / PP). In the surface treatment, silane used as another agent was aminopropyltriethoxysilane (APS). Glass fiber was treated with UB+APS, UB, or APS, and was applied to thermoplastic composites. In interfacial strength test, UB was effective in GF / PC, but ineffective in GF / PP. In order to clarify interfacial effect with UB on PC and PP, the diffusion of UB to resin was analyzed by microscopic FT-IR spectroscopy. As a result, the diffusion appeared in PC matrix, but not in PP matrix. In addition, APS depressed the diffusion of UB in PC including GF with APS+UB. These results suggested that the interfacial reinforcement needed not only the interaction between UB and resin, but also the interaction between silane and UB to produce the mechanical linkage from resin to fiber. This suggestion was supported from the interfacial strength in some other composites with matrix resin such as polybutylene terephthalate, polyamide 66, and copolymer of acrylonitrile and styrene.