The effect of various silane coupling agents on glass fiber surfaces has been studied in terms of the surface energetics of fibers and the mechanical interfacial properties of composites. gamma -Methacryloxypropyltrimethoxysilane (MPS), gamma -aminopropyltriethoxysilane (APS), and gamma -glycidoxypropyltrimethoxysilane (GPS) were used for the surface treatment of glass fibers. From contact angle measurements based on the wicking rate of a test liquid, it was observed that silane treatment of glass fiber led to an increase in the surface free energy, mainly due to the increase of its specific (or polar) component. Also, for the glass fiber-reinforced unsaturated polyester matrix system, a constant linear relationship was observed in both the interlaminar shear strength (ILSS) and the critical stress intensity factor (K-IC) with the specific component, gamma (SP)(S), of the surface free energy. This shows that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and coupling agents plays an important role in improving the degree of adhesion at the interfaces of composites.