Abstract: This is a report of theoretical and experimental researches on the stress relaxation of clay which play an important role in the phenomena of earth pressure. The assumption of a special model of clay is sufficient to express a stress relaxation which proceeds linearly with the logarithm of time and after a certain duration the stress relaxes to a final value. In the new type rheometer, the constant deformation is given to clay specimen and the relaxation of the internal stress is measured. Further, the thermal effect on stress relaxation of clay is discussed. Main results obtained here are as follows