This paper presents a comprehensive case study on electro-mechanical analysis of MEMS[1] variable capacitors. Using the fundamental mechanical and electrical equations, static and dynamic behaviors of the device are studied. The analysis is done for three different modes, namely: dc (static mode), small signal ac and large signal regime. A complete set of equations defining dynamic behavior of the MEMS, and an ac small signal equivalent circuit are presented. The mathematical models are defined and examined by the Matlab Simulink and a complete set of simulation results is reported for various cases separately. The results of this study would be useful in design and analysis of the MEMS based circuits which have some kind of mechanical dynamic action. Some examples of such devices may include VCO[2]s, frequency modulators, tunable filters and parametric effect circuits.