Abstract: Based on the knowledge of rheological characteristics of rocks, the strength degradation model is employed in the representative volume element to study the time-dependent deformation and failure of slopes in view of the physical essence that the mechanical properties of rocks are degraded with time and the meso-scopic damage accumulation due to the influence of environmental variation. The time-dependent deformation and failure of slopes with weak structural planes distributed along the slope surface is firstly simulated. The influence of spatial distribution and strength degradation characteristics of weak structural planes on the long-term stability of slopes is numerically investigated. Then the time-dependent failure of an existing slope with weak structural planes and alternatively distributed soft and hard rock layers is numerically investigated. Due to the rheological characteristics of rocks, the strength characteristics are degraded with time and the slope becomes more unstable. The long-term strength of weak structural planes and weak rocks plays an important role in the stability of rock slopes. The key factors such as long-term strength, seepage control and excavation-induced unloading should be thoroughly considered in the optimum design of slopes related to the safety of stabilization. The results obtained in this study may provide a meaningful guidance for the design and construction of other similar slopes.