A novel grain size dependent strain hardening model is derived from the theory of irreversible thermodynamics. The model yields the evolution of the dislocation densities in the grain interior and at the grain boundary, as well as their contributions to the flow stress. It is found that submicron grain sizes have a lower dislocation density in the grain interior, causing ductility to decrease greatly. The predicted stress-strain curve shapes, uniform elongation and ultimate tensile strength values for interstitial free steels (body centred cubic) and aluminium alloys (AA1100, face centred cubic) show good agreement with experimental observations.