||In this paper, we present a constitutive model for creep deformation in rock materials. Starting from an elastoplastic model for the description of short term behavior, the time-dependent deformation is described in terms of evolution of microstructure, leading to progressive degradation of elastic modulus and failure strength of material. The proposed model is applied to predict material responses in creep and relaxation tests. There is a good agreement between numerical simulations and experimental data. The proposed model is able to describe the main features observed in most cohesive frictional geomaterials (rocks and concrete), such as plastic deformation, damage, volumetric dilation, pressure sensitivity, rate dependency and creep.