Abstract: The underlying mechanism of creep deformation of rockfill dams is the particle deterioration due to environmental changes and the particle breakage under high contact stresses. A solid hardness parameter reflecting particle strength is introduced into the isotropic compress equation. In order to describe the deterioration process of rockfill particles with time, the solid hardness parameter is assumed to vary with time, following a mathematical representation of hyperbola distribution. By analyzing the data of triaxial creep tests on rockfill materials, a hyperbolic creep flow rule is established. The creep formulae for deformation are derived, and the expressions for creep modulus are proposed. Then a visco elastic-plastic constitutive model for rockfill materials is proposed considering the influence of both loading and creep deformation. Finally, the validity of the proposed model is confirmed by comparing the simulated results with the measured data of two kinds of rockfill materials.