Stochastic particle motion in the magnetic island in the presence of electrostatic perturbations has been studied both numerically and analytically. By evaluating the K-S entropy, it was shown that particle orbits can be stochastic by the simultaneous effects of magnetostatic perturbation and electrostatic ones with amplitude which are not large enough to induce the orbit stochasticity alone. The generation of orbit stochasticity is due to the overlapping of sub-island formed by the E B drift driven by electrostatic perturbations and is consistent with Chirikov criterion. For the given perturbed energy, the wide region in the particle orbit can be stochastic by the electrostatic perturbations with adjacent resonance surfaces and successive mode numbers, rather than the single perturbation with large amplitude. The particle diffusion process was also analysed by evaluating the cummulants of particle displacement in time and it was shown that the process is basically the Gauss one and the higher order cummurants that the second one are appeared due to the spatial inhomogeneity of the local diffusion coefficient. (author).