Abstract: In this paper, on the basis of conventional test results, an elasto viscoplastic constitutive equation is used to simulate the dilatancy behaviour and time effect for cohesive soils. The treatment of the dilatancy behaviour and time dependent stress strain relation according to the Perzyna′s viscoplastic equation and the non-associative flow rule is discussed and the computed results appear to be consistent with laboratory observations. The validity of proposed constitutive relations is confirmed by comparisons of the experimental observations with results of numerically simulated undrained triaxial tests.