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In this paper, a novel electret-based capacitive energy harvesting device has been developed according to out-of-plane gap closing scheme. The device is composed of a micro cantilever and a substrate which form a variable capacitor and is in series with a resistance. An electret material is used to provide the bias voltage which is needed in capacitive energy harvesters in order to scavenge energy from ambient vibration. The ambient vibration is applied to the system as a harmonic base excitation. The motion equations and the corresponding boundary conditions are derived using Hamilton' principle based on Euler-Bernoulli beam theory and the Kirchhoff's voltage law is employed to couple the mechanical and electrical fields. The equations of motion are discretized using Galerkin procedure and integrated numerically over time. Pull-in instability of the system is investigated in both static and dynamic cases. The effect of various parameters on the behavior of the device is studied. The maximum theoretical harvested power is resistance in the order of 1 W  .