An electrostatic pendulum oscillator was theoretically analyzed. In this theory, infinitely large electrodes are assumed, so that the electric force can be approximated by a constant for small angles of pendulum motion. The charge acquired by a sphere was calculated by both approximation and numerical method. Then the relationship between applied voltage and frequency was obtained, which might be approximated by a linear relation at higher voltage. The theory was compared with the experimental results of Martin and Longbrake. Even though the electrode configuration is different, the theory agreed reasonably well with the experimental data. Further improvement of the theory is suggested.