An ultrasonic sensor for monitoring a user's health characteristics, is flexible and close-fitting on the user's skin. The ultrasonic sensor includes a substrate, a signal transmitting layer positioned on the substrate, a signal receiving layer positioned on the substrate, and a flexible layer positioned on the signal receiving layer. The flexible layer is configured to attach to the user's skin. The signal transmitting layer includes a second electrode layer and a plurality of piezoelectric units formed on the second electrode layer. Each piezoelectric unit includes a second piezoelectric material layer formed on the second electrode layer and a conductive layer formed on the second piezoelectric material layer.
