In this paper, we have demonstrated the performance improvement of the air-borne Capacitive Micromachined Ultrasonic Transducers (CMUT) in transmit mode using the Helmholtz resonance principles. The natural frequency of the resonance cavity，and the Helmholtz resonance frequency of the aperture which is formed in the center of membrane, were matched for resonance during the design. In order to verify the theoretical concept, Finite Element Analysis (FEA) simulation was performed using COMSOL software. Through the simulation comparison with the traditional CMUT, the proposed CMUT demonstrates the feasibility to improve the output acoustic pressure and SPL of the air-borne CMUT in transmit mode by combining the mechanical resonance theory with Helmholtz resonance principle.