This paper deals with state estimation of micro tunable capacitor subjected to nonlinear electrostatic force. For this end a nonlinear observer has been designed for state estimation of the structure. Necessary and sufficient conditions for construction of the observer are presented. Stability of the observer is checked using Lyapunov theorem. Observer design is based on converting of differential equation of dynamic error from heterogeneous to homogenous. For this process non-linear electrostatic term is presented as coefficient of error and it is done using decomposition of Taylor expansion of non-linear term. By stabilizing of homogenous differential equation gains of observer can be obtained. Ability of the observer in state estimation of micro tunable capacitor is checked and related results are presented.