A compounded capacitive micromachined ultrasound transducer (CMUT) using two SOI wafers is proposed. This transducer has the potential in multi-frequency transducer applications and medical application for both imaging and therapy. The compounded CMUT (CCMUT) has a composite design with both low- and high-frequency structures built into one compartment of CCMUT. The effective parameters on collapse voltage including air gap, thickness and radius of the membrane in CMUT are simulated. The capacity in CMUT and electromechanical efficiency are also simulated. The critical parameters related including collapse voltage, capacity and electromechanical efficiency are analyzed and discussed. This CCMUT can be utilized in medical imaging, therapy or multi-frequency application.