Abstract: The interfacial debonding in FRP bonded repair will affect the bonding strength and lead to failure of the repair without warning. Unfortunately the interfacial damage is normally invisible and often in the form of a patch rather than a through-width crack. In this article, a debonding patch detection technique based on fiber optic interferometry is proposed. A quasi-impulse loading is applied with a rubber-head hammer and the total elongation of a surface-mounted optical fiber along the length of the repair material is measured as a function of load position. When a debonding patch is present, the induced sudden slope or sign change on the plot of fiber integral strain vs. load position will reveal the extent and location of the debonded area. The results of the study indicate that the proposed technique is applicable for debonding patch detection in repaired members under various support conditions.