In the design of devices containing beams of charged particles, it is helpful to know how the particle's trajectories will be modified by small displacement of electrodes. This paper describes a method to predict the perturbation of particle trajectories. In this methodology, the derivative of the potential is deduced from the results of finite element computation. The equation of motion is solved to calculate the perturbation of particle trajectories separately. An example of a perturbation trajectory in an electrostatic lens is presented.