The investigation of nanofluids effects on the performance of solar energy devices has converted to an important topic of research in recent years. The present experimental study deals with the effects of using WO3/water nanofluids on the efficiency of a flat plate solar collector which operates under weather conditions of Budapest, Hungary. First, water based nanofluids containing WO3 nanoparticles (with an average size of 90 nm) at three different volume fractions including 0.0167%, 0.0333%, and 0.0666% have been synthesized. The stability of nanofluids has been evaluated through Zeta potential tests which unveiled the prepared suspensions have high stability. In the next step, the thermal performance of the flat plate solar collector using nanofluids is investigated at different mass flux rates including 0.0156, 0.0183, and 0.0195 kg/s.m(2). The results showed that adding WO3 nanoparticles to water ameliorates the efficiency of the solar collector. The experiment results reveal that the maximum enhancement in efficiency of the collector at zero value of [(Ti-Ta)/Gr] was 13.48% for the volume fraction of 0.0666% and mass flux rate of 0.0195 kg/s.m(2) compared to water, which clearly shows the high potential of WO3 nanoparticles for solar energy applications. (C) 2017 Elsevier Ltd. All rights reserved.