Using nanofluids in thermal energy devices, such as flat-plate solar collectors, is gradually making progress, and getting awareness in the scientific community. Experiments were performed to study the effect of using CeO2-water on the efficiency of flat-plate solar collector by three different volume fractions of CeO2 nanoparticles of 0.0167%, 0.0333% and 0.0666%, while the mean particle dimension was kept constant at 25 nm. An ultrasonic process was used for maintaining the stability of the CO2-water nanofluid. The working fluid mass flux rates were 0.015, 0.018 and 0.019 kg/s m(2). The experiments were carried out in Budapest, Hungary on the latitude of 47 degrees 28'N and longitude of 19 degrees 03'E. Higher collector efficiency was achieved when using CeO2-water nanofluid compared to results achieved with water application. Based on present data, the efficiency of the collector is directly proportional with the mass flux rate and with the volume fraction in the ranges of the present study. Experiments indicate that the highest rise in efficiency of the collector at zero value of [(Ti-T-a)/G(T)] is 10.74%, for volume fraction (phi) 0.066%, and for mass flux rate of 0.019 kg/s m(2) compared to water.