The mechanical effects of the metal layer on the membranes of capacitive micromachined ultrasonic transducers (CMUTs) are analyzed in this paper by means of finite element simulations. The influence of electrode size and thickness on the electrostatic behavior of the single CMUT cell, including diaphragm displacement, cell capacitance, and collapse voltage, is explored. The effect on device sensitivity is investigated through the transformation factor of the cell, that is computed by FEM and compared with the parallel plate model prediction. It is found that for a non-negligible electrode thickness, as in the majority of fabricated devices, both the static and dynamic performance of the cell can be affected in a significant way. Thus, the effects of membrane metallization must be taken into account in CMUT design and optimization.