In the design of micro pressure sensors, the deflections of diaphragms are crucial to conduct performance simulations. This paper discuss two cases of diaphragm deformation in a three plates capacitive pressure sensor: a clamped edge diaphragm under uniform load and a cantilever plate deflects in touch mode operation. Analytical models are proposed to provide a quick estimation of the sensors. Evaluation of these models have been done by comparing the results to that of the finite-element method (FEM).