The current study on fiber reinforced plastics（FRP） bonded beam are conducted based on the assumption that the bond is very perfect between sheet and beam, and the thickness of adhesive is even. In fact, the thickness of adhesive is not perfectly even and continuous with the limitations of site and technology in construction. The present theory and failure mode about interfacial shear stress can not perfectly show the behaviors of bonded beams. This paper generally establishes the formula of shear stresses at bonded lactation in an FRP bonded beam considering non-linear behavior of concrete under compression and steel rebar, and compares the calculation results of with those of FE. The result shows that they are approximate.