A piezoelectric ultrasonic sensing system based on an optical fiber has been developed for detection of various damages. The ultrasonic wave generated from a piezoelectric actuator is guided and propagated in the optical fiber and then sensed by a piezoelectric sensor located at the other end of the fiber. The sensed signal can be influenced by environmental and physical changes around the optical fiber. In this study, the sensitivity of the optical fiber sensor is experimentally studied. Various patterns of damage, such as cracks, loosen bolts, holes, are more common forms in real structures. In particular, detection method of the damage with respect to various depths of the crack is presented in this paper.