High-Q factor voltage controlled oscillators (VCO) need a wide tuning range and low phase noise over gigahertz ranges of frequency which depends on the tunability of the capacitors in the LC tank circuit. The reasons behind the development of a micro electro mechanical (MEM) varactor were the difficulties encountered in the realization of on-chip variable capacitors having low phase noise and high quality factors with a wide tuning range in the span of frequencies over process and temperature variations. This paper presents an efficient closed-form model for determination of the pull-in voltage in a surface micro machined MEM varactor which is a factor directly affecting the tunability of the device. The nonlinear spring hardening effects associated with proper load-deflection characteristics of clamped plates and the electrostatic spring softening effects due to the parallel-plate and fringing field capacitances have been taken into account with the dimensions of the device optimized through finite element analysis (FEA).