A prototype of CuO nanofluid-based Direct Absorption Solar Collector (DASC) was built with applicability for domestic solar water heater. Based on the transmittance spectra of the nanofluid samples, light is not able to pass through the sample in the volume fractions more than 100 ppm. The effect of collector internal surface of bottom wall, fluid inlet temperature and flowrate and CuO nanoparticles volume fraction on the efficiency of the collector is experimentally studied. The collector efficiency increased by increasing nanofluid volume fraction and flowrate; however, this increase has asymptotic trend. The results show that zero-loss (maximum) efficiency of the collector for 90 l/h and nanofluid with 100 ppm volume fraction is highest; so that, the collector efficiency is about 17% more than that of using the base fluid as working fluid at similar flowrate.