An experimental study performed to investigate the effect of Cu nanoparticle on the efficiency of a flat-plate solar collector. The weight fractions of the nanoparticles tested in this study having average diameter of 10 nm, were 0.2% and 0.3% of the nanofluid. A one-step method was used to prepare copper nanofluid from reduction of CuSO4 center dot 5H(2)O with NaH2PO2 center dot H2O in ethylene glycol as the solvent. The experiments were performed in different volume flow rates of the nanofluid from 0.016 to 0.050 kg/s and the standard of ASHRAE 93 was used to test the solar collector's performance. It was found that by increasing the nanoparticle weight fraction, the efficiency of the collector was improved. Also, the lowest removed energy parameter could be reached by using 0.3 wt% Cu/EG nanofluid at 1.5 Lit/min and the highest absorbed energy parameter was achieved under the same conditions. (C) 2014 Elsevier Ltd. All rights reserved.