The partial specific Gibbs1 free energy or the chemical potential has been shown to be the most desirable thermodynamic function for describing soil-water systems. The absolute value of the chemical potential of soil water cannot be determined. If pure water at standard temperature and pressure is taken as the reference, the difference between the chemical potential of soil water and that of the reference can be measured and is called "soil water potential," which has the dimension of energy per unit mass. The barometric pressure at the sea level has been accepted as the standard reference pressure. Thus, any device that measures a certain property with respect to local barometric pressure should be standardized to the pressure at sea level. In case of a tensiometer, a device for measuring soil water potential operating at different locations with varying altitudes, the reference pressure changes from place to place and time to time. In pressure plate or pressure membrane apparatus, on the other hand, the soil water potential, there would be a problem with respect to pressure in evaluation of water potential. The effect of external pressure on soil water potential has been neglected by most workers in the past. That is, they have assumed that soil water behaves the same way toward pressure as pure free water. The main objective of this investigation is to evaluate the validity of this assumption and to determine the nature of the pressure dependence of soil water potential.