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An experimental investigation was conducted aiming to determine the heat transfer, friction and instantaneous collector thermal efficiency of a thermosyphon (natural circulation) solar water heating system using as working fluids of water and the Cu/H2O nanofluid. The Cu nanoparticles were synthesized using the chemical reduction method and characterized by the x-ray diffraction and transmission electron microscopy techniques. The stable Cu/H2O nanofluid was prepared for the volume concentrations of 0.1% and 0.3%. The empirical correlations developed for Nusselt number and friction factor for the Cu/H2O nanofluid fit the experimental data with a deviation of less than ±3.5% and ±2.5%, respectively. The results of present experimental investigation were presented at various Reynolds number and particle volume concentrations under thermosyphon conditions. The comparison indicates that the heat transfer enhancement obtained with the Cu/H2O nanofluid for the thermosyphon is higher than that for the plain tube collector and increases with the increase of particle volume concentration. The overall thermal performance of the thermosyphon increases when the operating fluid is Cu/H2O nanofluid as compared to water.