Flat plate solar water heater is widely used for heating of water in low-temperature residential applications. In this paper, Copper Oxide/water (CuO/H2O) nanofluid is prepared from Copper Acetate and its thermal performance was investigated experimentally on a 100 Liters per Day (LPD) thermosyphon based indirect-type flat plate solar water heater. The volumetric fraction of CuO/water nanofluid chosen was 0.05%. Significant improvement in performance was observed in thermosyphon circulation compared to forced circulation, for the low volumetric fraction considered. The CuO/water nanofluid was prepared with the inclusion of surfactant Sodium Dodecyl Benzene Sulfonate (SDBS), as it provided the best CuO nanoparticle dispersion stability compared to pure water suspension and Triton X-100 surfactant suspensions. Also, the thermophysical properties of the synthesized nanoparticle and prepared nanofluid were compared theoretically and experimentally. (C) 2015 Elsevier Ltd. All rights reserved.