Abstract: The present work aims at developing a fiber optic based technique to detect delamination at the web/flange junction of a GFRP I-beam based on the frequency domain analysis of ultrasonic echoes. A PZT transmitter was employed to create the stress waves in the beam element, and an optical fiber interferometric sensor was attached to the surface of the beam to serve as an ultrasonic receiver. From the frequency spectrum of the sensor output corresponding to various positions of the PZT transmitter, the delamination location can be revealed. Based on the results from both theoretical and experimental studies, the feasibility of delamination detection with the proposed technique is demonstrated.