Abstract: The rockfill materials have been widely used in the construction of rockfill dams due to their high strength, small deformation and strong permeability. The strength and deformation characteristics of the rockfill materials are essential prerequisites to the design of rockfill dams. The existing laboratory tests have shown that the stress-strain relation of the rockfill materials is closely related to their states in density and pressure. Within the framework of the critical state and bounding surface plasticity theories, a state-dependent 3D multi-mechanism bounding surface model is proposed for the rockfill materials by considering their nonlinear characteristics in deformation and strength. The macroscopic deformation behaviors of the rockfill materials in this model are decomposed into a macroscopic volumetric mechanism and a set of independent virtual microscopic shear mechanisms in spatially distributed orientations. Each microscopic shear mechanism is described by the microscopic shear stress-strain relations and microscopic stress-dilatancy relations in three directions. A state parameter is introduced in the strength criterion and stress-dilatancy relation for compatibility with the critical state theory. Some relations are established between the microscopic and macroscopic model parameters. The model has twelve parameters, and most of them are of clear physical meanings. The proposed model is used for simulating the triaxial compression tests on two types of rockfill materials. The results show that the calculated values are in good agreement with the test data, indicating that the proposed model is capable of predicting reasonably the strain-hardening and strain-softening behaviors of the rockfill materials under different densities and confining pressures.