We have investigated the effect of natural frequency shift in single-crystal silicon, rectangular, electrostatic torsion actuators due to the electrostatic force. The electrostatic actuator was fabricated by using ultra-thin silicon wafer, SU-8 thermal compression bonding, and deep reactive ion etching. Natural frequency and Q factor of the actuator were tested in a high vacuum ambient. An analytical model was proposed to simulate the effect of natural frequency shift due to the electrostatic force. It showed that the natural frequency decreased with the increase in applied voltage, which could be used to tune the natural frequency of the torsion actuator. At a voltage near pull-in, the natural frequency was close to zero due to the negative electrostatic spring constant. The pull-in curve of the actuator was tested in air. Good agreement was found by comparing the results of analytical model with those of Coventor 2001 finite element analysis software and experiments.