Abstract: The volumetric strain due to stress dilatancy of sand is found to be composed of a reversible dilatancy component and an irreversible dilatancy component. The former is characterized by its reversibility and dependency on the magnitude and direction of the current shear strain, and conversely, the latter by its irreversibility and dependency on past shear history. New mechanisms and formulation have been developed for description of the reversible and irreversible dilatancy. Influence of the irreversible dilatancy component on the stress strain response was also investigated based on new experimental observations.