We present a highly sensitive fiber-optic ultrasonic sensor based on π-phase-shifted Fiber Bragg gratings (πFBG) for structural health monitoring. Due to the phase discontinuity at the center of the grating, the reflection spectrum of a πFBG features a narrow notch, leading to significantly improved sensitivity over traditional fiber-Bragg-grating based ultrasonic sensors. The πFBG sensor interrogation is based on a wavelength-tunable, narrow-linewidth diode laser whose wavelength is locked to center of the linear slope of the spectral notch. Two different fabrication methods of πFBG are developed. Highly sensitive detection of ultrasonic waves on an aluminum plate using a πFBG sensor is demonstrated and its performance is compared to a PZT sensor system. The directivity of a πFBG sensor is also characterized. The noise performance of the πFBG sensor system is theoretically analyzed.