The　interfacial　fatigue　damage　behaviors　of　two　reinforced　concrete　(RC)　beams　strengthened　with　fiber　reinforced　plastic　(FRP)　were　recorded　with　the　infrared　detecting　method,　and　based　on　a　series　of　fatigue　tests　the　interfacial　fatigue　properties　and　interfacial　fatigue　lives　were　studied.　It　is　found　that　the　infrared　detecting　method　can　be　used　to　monitor　the　interfacial　debonding　effectively　because　the　difference　in　infrared　temperature　between　the　bonding　and　debonding　areas　is　obvious.　The　interfacial　debonding　initiates　at　the　concrete　cracks　around　the　mid-span　and　then　expands　to　one　end　of　the　beam,　in　which　the　process　for　the　initiation　and　stable　or　unstable　growth　of　the　interfacial　debonding　has　been　observed　with　the　stable　phase　about　99%　of　the　total　interfacial　fatigue　life.　In　the　main　fatigue　phase,　the　interfacial　damage　is　little　and　increases　slowly;　in　addition　FRP　stress　changes　smoothly,　showing　that　the　interfacial　fatigue　properties　are　stable.　The　use　of　FRP　high　strength　is　restrained　by　the　interfacial　fatigue　failure,　and　at　last　according　to　test　data　a　prediction　approach　for　interfacial　lives　is　given.