A piezoelectric ultrasonic sensing system based on an optical fiber has been developed for two-phase flow monitoring. 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. For this study, the sensitivity of the optical fiber sensor is experimentally studied according to the system parameters, such as fiber coating, frequency, and the number of cycles of the input voltage signal to the piezoelectric actuator. The developed sensing system is applied for monitoring the void fraction of the two-phase flow and the feasibility as a two-phase flow monitoring system is demonstrated.