In this paper, an analytical study is performed on the entropy generation and heat transfer due to nanofluid flow in a flat plate solar collector. The working fluid considered in this work is Al2O3/water nanofluid with four different particle sizes, including 25, 50, 75, and 100 nm and volume concentrations up to 4%. Effects of tube roughness, nanoparticle size, and different thermophysical models are investigated on the Nusselt number, heat transfer coefficient, outlet temperature of the collector, entropy generation, and Bejan number. In addition, the effects of solar radiation and ambient temperature on entropy generation are examined. The results are presented for constant mass flow rates ranging from 0.1 to 0.8 kg/s. It is found that when the mass flow rate is considered to be constant for all working fluids, the Nusselt number and heat transfer coefficient have different trends. It is observed that uncertainties in thermophysical models and tube roughness have considerable effects on the values of heat transfer coefficient and Nusselt number. The findings show that with an increase in the volume fraction of nanofluid, the outlet temperature increases while with increasing the nanoparticle size a very insignificant decrease is observed in the outlet temperature. It is seen that the trend of changes in the outlet temperature is exactly in opposite to the Nusselt number trend. The analysis of entropy generation concludes that the entropy generation decreases with increasing the nanofluid concentration. It is found that the tube roughness increases the entropy generation and its effect is more visible at high mass flow rates while the effects of uncertainties in thermophysical models on entropy generation are not significant in any mass flow rate and volume fraction. Finally, a critical mass flow rate is determined under two different intensities of solar radiation and ambient temperature so that for the values higher than the critical mass flow rate the effects of roughness on entropy generation become important and should be considered. (C) 2014 Elsevier Ltd. All rights reserved.