The strengthening of reinforced concrete structures by means of externally bonded fiber reinforced polymers FRPs is becoming an attractive technique for upgrading existing structures. Although previous laboratory investigations have shown that the shear capacities of beams can be increased considerably with this strengthening technique, premature failure by debonding of the FRP reinforcement can often limit its effectiveness. To gain insight into debonding phenomena, various experimental and analytical investigations of the behavior of bonded FRP-to-concrete joints have been carried out. However, such studies have generally been limited to monotonic "static" loading conditions. The primary purpose of this paper is to propose a new methodology of determination of stiffness and degradation of stiffness of the interface between CFRP side face and concrete under cyclic loading. The test setup and supports are designed such that there is a direct shear at the FRP-to-concrete interface. In this paper, first, we describe the experimental setup and test parameters. Next experimental results for cyclic bond stress-slip behavior, mathematical formulations for stiffness of interface and its degradation as number of cycles increased are obtained and discussed.