Photovoltaic/thermal (PV/T) system is a kind of solar system that converts solar energy to electrical and thermal energy simultaneously. In this paper, the effect of some parameters, such as packing factor, fin number, and fin height as well as environmental and dimensional parameters on the performance of the PV/T system with natural air flow from the energetic and exergetic viewpoint, has been studied. For this purpose, a theoretical model is developed and validated. Induced mass flow rate and PV temperature are well-predicted, compared with existing numerical data, available in the literature. It is found that thermal efficiency of the PV/T systems decreases slightly with the increase in packing factor, while PV efficiency increases sharply. The first-law efficiency and second-law efficiency of the system increase with the increase in the fin number and fin height. Also, results show that increasing exit diameter is a favorable factor from the first-law efficiency viewpoint, while it has an unfavorable effect on the second-law efficiency.