This paper investigates the dynamic behavior of a microbeam-based electrostatic microactuator. The cross-section of the microbeam under consideration varies along its length. A mathematical model, accounting for the system nonlinearities due to mid-plane stretching and electrostatic forcing, is adopted and used to examine the microbeam dynamics. The Differential Quadrature Method (DQM) and Finite Difference Method (FDM) are used to discretize the partial-differential-integral equation representing the microbeam dynamics. The resulting nonlinear algebraic system is solved for the limit cycles of various microstructure geometries under combined DC-AC loads and the stability of these limit cycles is examined using Floquet theory. Results are presented to show the effect of variations in the geometry on the frequency-response curves of the microactuator. We examine the effect of varying the gap size and the microbeam thickness and width on the frequency-response curves for hardening-type and softening-type behaviors. We found that it is possible to tune the geometry of the microactuator to eliminate dynamic pull-in.