A capacitive micromachined Lamb wave transducer using the second-order bending mode of high-aspect rectangular diaphragms to excite and receive Lamb waves in a silicon plate was presented [L. Ge, Proc. SPIE 6358, 1 (2006)]. Thus, the width of the diaphragm is made equal to the wavelength of the A0 wave, and all the transducer parameters can be determined by the TDK model for CMUTs with high-aspect rectangular diaphragms. The sensitivity, i.e., a ratio of the relative frequency shift to the change in mass loading of the plate, is derived and is a nonlinear function, which is the same as that for capacitive ultrasonic mass resonators [L. Ge, IEEE IUS, 1034 (2008)]. Also, it is a general nonlinear sensitivity expression available for both capacitive and piezoelectric gravimetric sensors. The area density increment, caused by the adsorbed or bound molecules or the fluid loading, is usually comparable with the area density of the silicon plate. Therefore, it is significant to use the nonlinear expression for determining the sensitivity accurately. The capacitive Lamb wave transducers are very attractive as gravimetric sensors for chemical and biological sensing since they have much simple structure and high sensitivity. [Work supported by NSFC (60774053).]