In this work, analytical and experimental studies on thin bilayer membrane-type Capacitive Micromechanical Ultrasonic Transducer (CMUT) have been carried out, and results are reported and discussed. The bilayer membrane, consisting of two layers — one of PMMA and the other of graphene — is the key component of the designed CMUT. Because of its high diameter/thickness ratio and relative thinness, the PMMA/graphene membrane primarily relies on tension-induced restoring force. A membrane-type model is constructed for the bilayer circular membranes and the PMMA/graphene composite membrane’s vibrational properties have been determined from the motion characteristics of rectangular and circular shaped membranes. The outcomes of the FEA simulation validate the accuracy of the theoretical analysis. A CMUT based on an ultra-thin PMMA/graphene membrane was designed and fabricated by combining graphene with PMMA to form a bi-layered structure. Subsequently, a CMUT based on the PMMA/graphene composite membrane was developed. The fundamental frequency of the fabricated device is 1.69 MHz, closely agreeing with theoretical predictions based on simulation results.