Abstract: A series of triaxial compression tests were performed to investigate the deformation characteristics and critical state of sand. The test results show that subjected to shear, a sand will exhibit either dilative or contractive behavior. Whether a sand dilates or contracts depends on the current state of the sand, that characterized by both the density of the sand and the effective mean normal stress applied. The larger the density, the lower the effective mean normal stress, the more dilative response is. It is also found that at large strain levels, critical state will appear in all the soil samples tested, and the critical state line is unique irrespective of drainage condition.