To investigate the convective heat transfer of nanofiuids, experiments were performed using silver-water nanofiuids under laminar, transition and turbulent fiow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass fiow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silverwater nanofiuid, with ±10% agreement between experiments and prediction. To investigate the convective heat transfer of nanofiuids, experiments were performed using silver-water nanofiuids under laminar, transition and turbulent fiow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass fiow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the based on the experimental results a correlation was developed to predict the Nusselt number of the silverwater nanofiuid, with ± 10% agreement between experiments and prediction.