The ultimate relative slip between reinforced polymer (FRP) and concrete on their bond interface is a key parameter to analyze the debonding failure of FRP strengthened reinforced concrete beams. Lots of simple shear test specimens have been used to test the slip-shear stress relationship on the bond interface. However, the ultimate FRP-concrete slip has not been obtained directly from these experiments. Based on different theoretical considerations some ultimate slip values have been predicted. These values differ from one another to a large extent. The lower limit of the maximal slip between FRP and concrete in the span of strengthened beams is derived through a set of simple assumptions. Example calculations show that at loads exceeding 80% of the designed loads most maximal slips would reach 0.2mm or greater. The fact that most experimental beams at such loads do not exhibit debonding failure manifests that the ultimate slip should be greater than 0.2mm.