A phase mixture model is applied to describe the deformation behaviour of nanocrystalline copper at room temperature. The deformation behaviour of the grain interior 'phase' is described by constitutive equations based on dislocation and diffusion controlled plasticity. The grain boundary 'phase' is considered to deform by a proposed diffusional mechanism. The effects of strain rate and grain size on the deformation behaviour of nanocrystalline copper were investigated. Observed deviations from the Hall-Petch behaviour are described by the model correctly.