Mixed convective heat transfer of water/alumina nanofluid inside a vertical microchannel is investigated theoretically. A modified Buongiorno's model is employed for the nanofluid, which fully accounts for the effect of the nanoparticle migration. This model considers the Brownian motion and thermophoresis diffusivities as the predominant slip mechanism. Because of surface roughness in microscale channels, slip condition is considered at the walls, which appropriately represents the hydrodynamic boundary condition. The results obtained indicated that nanoparticles move from the heated walls (nanoparticle depletion) toward the core region of the channel (nanoparticle accumulation) and construct a non-uniform nanopartide distribution. In addition, increasing the bulk mean volume fraction of nanoparticles phi(B), slip parameter lambda and mixed convective parameter Ng enhances the heat transfer rate. Moreover, in contrast to lambda, phi(B) and Ng have a negative effect on the pressure drop of the system. (C) 2014 Elsevier B.V. All rights reserved.