Abstarct: In this study, the effect of CuO-water nanofluid, as the working fluid, on the efficiency of a flat-plate solar collector is investigated experimentally. The volume fraction of nanoparticles was 0.4% and the particle dimension was 40nm. The mass flow rate of working fluid varied from 1 to 3 kg/min. The ASHRAE standard was used to calculate the efficiency. The results show that, in comparison water as absorption medium using the nanofluid as working fluid increase the efficiency. For 1 kg/min of mass flow rate the increased efficiency was 21.8%. From the results it can be concluded that the choice of optimum mass flow rate causes increase in efficiency.