As one of structural health monitoring systems, the authors developed a broadband ultrasonic propagation system using macro fiber composite (MFC) actuators and fiber Bragg grating (FBG) sensors. This system can send and receive broadband Lamb waves efficiently in a specific direction, and the MFCs and FBGs can be integrated into composite laminates because of their flexibility and high fracture strain. In this research, this system was applied to detect an interlaminar delamination in carbon fiber reinforced plastic (CFRP) laminates. One MFC and one FBG were bonded on each surface of the CFRP laminate to generate and receive the symmetric modes and the anti-symmetric modes separately for identification of the multiple modes of Lamb waves in the laminate. Then we investigated the mode conversion of Lamb waves at both tips of a delamination in the middle of the thickness of the laminate, through an experiment and finite element analysis (FEA). On the basis of the knowledge about the mode conversion, an index to evaluate the delamination length was proposed using velocity changes by the mode conversions. From the results of experiments and FEA for the laminates with an artificial delamination, this new index was found to be effective to evaluate the delamination progress in the laminate quantitatively.