Evolution of Liesegang patterns in an external electric field was studied numerically using a discrete stochastic model and by real experiments. In the stochastic model the diffusion was described by a Brownian random walk of discrete particles. The precipitation reaction was regarded to be a stochastic process as well, and its description was based on Ostwald's supersaturation theory. Our real experiments and the results of this stochastic approach have shown a good agreement. On the basis of our results we have proposed an extended form of the width law, which takes into account the effect of constant electric field.