A 3×3 coupler based Mach-Zehnder Interferometer is used for delamination patch detection in FRP composite. Mechanical tests are performed and the sensing arm of the interferometer is embedded along the neutral plane of the laminate. A quasi-impulse loading is applied and the total elongation of the embedded optical fiber is measured as a function of load position. When a delamination is present, the shift of neutral axis position can be clearly reflected in the fiber integral strain versus load position plot. Experimental results demonstrate the feasibility of detecting both the size and location of delamination patch with the proposed technique.