Hybrid PV- thermal system (PV/T) is used to harvest both electrical and thermal energy from solar radiation. Using a hybrid PV/T system improves the electrical efficiency of the PV module while the thermal energy can be used for various heating applications. The aim of this work is to numerically study the performance of the PV/T system in a hot climate condition. The cooling system of PV/T consists of several channels parallel to each other. A novel three-dimensional numerical simulation of the system was performed to study the thermal efficiency. The standard k-epsilon turbulence model was used during the simulation. This novel system was based on applying different thermal boundary conditions on the sides of one channel rather than simulating the whole cooling system. Using one channel simulation allows the creation of a smaller number of meshes, hence, reduces the computational time. The thermal efficiency of the PV/T system was calculated using the superposition method for different mass flow rates. The results showed an increase in the thermal efficiency from 60% up to 68% by increasing the mass flow rates from 0.4 - 5.4 L/min. These results were in good agreement with the results in the literature.