A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m x 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m(2) and the inlet temperatures were controlled in the range of 35-65 degrees C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fluid. In our experiments, for 10,000 ppm concentration of silver nanoparticles, the optical characteristic and the thermal loss characteristic of the solar collector, under steady-state condition with a mass flux of 1.2 kg/min-m(2), were 0.691 and 4.869 W/m(2)-K, compared with 0.684 and 7.178 W/m(2)-K, respectively for 1,000 ppm concentration and 0.702 and 8.318 W/m(2)-K for water. When the flow rate was different from the standard value, the solar thermal characteristics were also improved with the nanofluid.