In this paper, a Mach-Zehnder based fiber optical sensor enabling to detect the acoustic waves is reported. Reported sensor is capable to sense ultrasonic waves from 10 to 80 kHz in an ambient condition and high frequencies up to 310 kHz in a water tank. It is anticipated the sensor have the ability for detecting ultrasonic wave caused by partial discharge in semi-insulating liquids like impaired tranformer oil.