Based on the existence of a yield surface and a memory surface which represents the maximum stress state the material ever experienced, a plasticity model is proposed to describe the inelastic behavior of metal. By choosing the initial locations and sizes of these two surfaces based on the internal structure and the loading history of materials, theoretical results have been obtained. These results compare well with available experimental observations on cyclic creep, cyclic stress relaxation and other transitory phenomena of strain softening materials under generalized cyclic loading conditions. The inelastic behavior of annealed copper subjected to nonproportional strain paths can also be predicted by the proposed model.