To improve the interfacial adhesion at interfaces between fibers and matrix, the γ-methacryloxypropyltrimethoxy silane (90 wt%, MPS) containing γ-aminopropyltriethoxy silane (10 wt%, APS) was applied to the surface treatment of glass fibers with different concentrations. The contact angle of silane-treated glass fibers and interlaminar shear strength and critical stress intensity factor (K_IC) for mechanical interfacial properties of glass fibers/unsaturated polyester were measured. According to the contact angle measurement, the silane-treated glass fibers did lead to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. And mechanical interfacial properties of the composites were improved in the case of silane-treated composites compared with those as received and were shown in a maximum value in the presence of 0.2 wt% MPS/APS, due to the peptide bonding between CO of MPS and –NH of APS. It revealed that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and the coupling agents plays an important role in improving the degree of adhesion at interfaces in a composite system.