Electrostatic field can change the pressure of dielectric system, and this effect is related to the exerted direction of the electric field. With this characteristic, enhancing mass and heat transfer can be achieved by applying external electric field. In this paper, based on the laws of thermodynamics, the fundamental equation of the dielectric system with the influences of electrostatic field was established. The mechanical equilibrium condition was deduced from this fundamental equation by the free energy criterion for thermodynamic equilibrium state, which is the effective pressure of each phases must be equal to each other. This effective pressure includes the mechanical effect of the electrostatic field on the dielectric system. It consists of two parts, real pressure within the system and the electric tension on the interface between phases, which leads to the difference of the real pressure within different phases. For a two-phase system with a planar interface, when the electric field is parallel to the phase interface, the real pressure within the phase with larger dielectric constant is larger; when the electric field is perpendicular to the phase interface, real pressure within the phase with larger dielectric constant is smaller; and while the field acts on part of the system, the real pressure in the region with electric field is higher than that without electric field. With this characteristic, the vapor pressure of gas-liquid system could be reduced with exerting electrostatic field along the direction parallel to the interface, or it could be increased when the exerting direction is perpendicular to the interface. Our conclusions derived from equilibrium thermodynamics are consistent with the electrohydrodynamic results. (C) 2017 Elsevier Ltd. All rights reserved.