For meticulous calibration of ultrasonic probes, hydrophones are usually used but it is difficult to find hydrophones that can operate up to 30 MHz with a very large bandwidth. Our group has a long experience in fabrication of the new cMUT transducers; these devices can easily operate in the high frequency range due to the silicon micromachining technology used. In order to calibrate large bandwidth high frequency cMUT array probes, we designed and fabricated a single element transducer on the same wafer used for the cMUT array. Since the sensor is fabricated on the same wafer, it has the same characteristics of the probe and, hence, the measurements are possible in the same operative range (3-20 MHz). Our initial results for cMUT single-element sensor are promising