In this paper, a numerical study is performed to investigate the effects of nanofluid on the productivity of a stepped solar still. Moreover, a sensitivity analysis is arranged to determine the sensitivity of the hourly productivity to the height and length of steps. Finally, an optimization analysis is performed by using response surface method to optimize the geometry of stepped inside the still. Obtained results indicate that 22% enhancement in the hourly productivity is observed by increasing the nanoparticle concentration from 0% to 5%. Moreover, there is only 2.1% difference between the estimated results by RSM and calculated results by CFD. (C) 2017 Elsevier Ltd. All rights reserved.