Solar energy as a renewable energy source can be used to produce heat and electricity. Among solar energy power production systems, photovoltaic thermal (PV/T) technology is almost a new and widely-used hybrid technology that can simultaneously produce heat and electricity. In recent years, nanomaterials in solar energy systems have significantly increased. In the present study, the effect of using nano-fluid water-aluminum oxide at three volumetric concentrations (0.10, 0.07, and 0.05%), and three inlet flow rates (0.6, 0.9, and 1.2 l/min ) on the electric efficiency, thermal efficiency and exergy efficiency of PV/T system at different times of the day were numerically calculated using CFD. Then, their results were compared with the observed experimental results. The inlet flow rate of 1.2 liters min-1 and the aluminum oxide nano-fluid concentration of 0.10% were optimized as input points due to the proper cooling of the system. The thermal efficiency is a function of the difference between the inlet and outlet temperatures of the fluid, and its highest value was observed at the maximum irradiation condition (960 W/m2 ) with 53%. Additionally, the highest and lowest exergy efficiency values were 21.6% and 15.9%, respectively, in the afternoon.