Strengthening of reinforced concrete (RC) structures by external bonding of advanced fiber-reinforced polymer (FRP) composites has become very popular around the world over the past decade due to the well-known unique advantages of FRP composites over other materials. Such FRP-strengthened structures are likely to fail by debonding of the FRP from concrete. This study concerns with understanding and modeling of debonding failure mechanism in FRP strengthened RC beams. For this purpose ,first the literature was reviewed to recognize reasons and mechanism of debonding. Then, the obstacles involved in nonlinear modeling of RC beams strengthened with FRP sheets were investigated. ABAQUS software capabilities were utilized to overcome the modeling difficulties. Special procedure was selected to model the interface between the FRP laminate and the concrete substrate; then the procedure wa verified with experimental results of six concrete beams strengthened with FRP and loaded up to debonding failure of laminates. A comprehensive case study was performed to access the effects of different geometrical parameters like length, width and height of the FRP laminate on the failure load and the whole behavior of strengthened beams. Finally, a new strength method was presented to predict the debonding load of the strengthened beams. Comparisons between the predictions of the presented method and other methods in the literature on some experimental data showed the good accuracy of suggested method.