The bonding of fiber reinforced polymer (FRP) plates or sheets to the concrete structures has been found to be an effective technique to improve the capacity. As a result, a large number of studies have addressed debonding failures in FRP-strengthened RC structures, with many of them being focussed on understanding the behaviour of simple FRP-to-concrete bonded joints in which an FRP plate/sheet is bonded to a concrete prism and is subject to a tensile force. With the help of features of FRP–concrete interface and experimental data in the literatures, a non-linear mode II interface law is presented. The proposed interface law includes non-linear contributions of adhesive and concrete cover at high shear stresses. The numerical results presented in this study show that FRP strains, shear stresses, slips and values of delamination force in the bonded region are in good agreement with experimental results. The agreement verifies the validity of the proposed interface law.