Abstract: 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.