Forward bias capacitance-voltage characteristics and interface state density of Au/n-GaAs Schottky diode were investigated by capacitance technique under hydrostatic pressure. It is shown that the capacitance-voltage characteristics are influenced due to the change of ionised additional donor-like defect centres. The interface state density decreases with the increase of energy from the edge of the valence band. In addition. it has been found that the hydrostatic pressure seriously affects the interface state density.