Abstract: Based on the field investigation, this paper deals with the geological features and formation mechanism of a large-scale landslide in a hydro-electric power station',reservoir area in the upreach of Yellow River. The landslide developed in a double-decked slope composed of soft rockmass in its lower part and hard rockmass upper parto It is demonstrated that the evolution of slopes of this kind is always initiated by the compression of the lower soft rockmass, then leading to the tension fracture and toppling of the upper hard rockmass, and at last the landslide of creep sliding-tensile fracturing type would be along the shear stress concentration zone in the slope.