This paper focuses on a non-destructive testing technique, infrared thermography, which is used to detect delaminations in concrete structures strengthened by CFRP jacketing. Four Specimens were designed and manufactured based on an existing bridge strengthened by CFRP jacketing. The conditions of the artificial delaminations in specimens, including size and depth, presence of surface cover mortar, and water content in the delamination void, were considered as test parameters. FEM simulations by using the Finite Element Model were also conducted and the limit of detectable delamination was discussed according to parametrical analysis. The results show that the existence of cover mortar, the size and depth of delamination have the most marked influence on the detectability, and the distinction of delamination was difficult when the delamination voids are fulfilled with water. The results of simulation and experiment show good agreements, and the lower limit of detectable size was presented.