Fine-grained materials produced by equal-channel angular pressing (ECAP) exhibit kinematic hardening due to the existence of a back-stress. This article presents a new dislocation-based model, which is able to describe the tension/compression asymmetry of the ECAP processed commercial purity aluminum. By introducing strain relaxation, and relating the back-stress to the inhomogeneous dislocation density distribution in cell walls and in cell interiors, the model can accurately predict the evolution of the dislocation densities, the cell size, and the back-stress. Compared to the other back-stress models, it takes into account the microstructure evolution and gives a better prediction.