This paper predicts the changes in the spring constant, the resonant frequency, the electrostatic force, and the displacement of a resonant structure due to non-ideal anisotropic RIE etching process. First, a 6 ㎛ thick polysilicon was etched by RIE and the anisotropy of the etched structure was measured as a function of a RF power, a Cl₂ flow rate and a chamber pressure. In the experimental results, an anisotropy was decreased as the RF power, the Cl₂ flow rate, or the chamber pressure was increased. A comb actuator`s operation characteristic was predicted depending on the anisotropy variations in RIE etching. Comb actuators with three different support beam structures were investigated : fixed-fixed, crab-leg, and double crab-leg. As the RIE etch anisotropy becomes non-ideal, i.e. the cross section becomes rather a trapezoidal than a rectangular shape, it decreases spring constant, resonant frequency and electrostatic force of a comb actuator but it increases the displacement of the mass. Among the three structures, the comb actuator with double crab-leg support beams is more influenced by anisotropy variation in RIE etch than other two.