The purpose of this work is to model the mechanical behavior of nanocrystalline materials. Based on previous rigid viscoplastic models proposed by Kim et al. (Acta Mater, 48: 493, 2000) and Kim and Estrin (Acta Mater, 53: 765, 2005), the nanocrystalline material is described as a two phase composite material. Using the Taylor-Lin homogenisation scheme in order to account for elasticity, the yield stress of nanocrystalline materials can be evaluated. The transition from a Hall-Petch relation to an inverse Hall-Petch relation is defined and is related to a change in plastic deformation mode in the crystallite phase from a dislocation glide driven mechanism to a diffusion-controlled process.