Abstract: The sensing arm of a fiber optical Mach-Zehnder Interferometer is embedded into the middle of composite beams to detect the delamination in the beams. Good results are obtained showing the delaminations can be detected easily whether the beams are simply supported or continuously supported. By pressing the beam continuously using a hammer, the output curve can be obtained from the fiber optical interferometer. One can judge if there exists delamination, and where the delamination is from the curve exactly.