Photovoltaic/thermal (PV/T) systems refer to the integration of photovoltaic and solar thermal technologies into one single system, in that both useful heat energy and electricity are produced. With the increased use of PV/T systems, more attention has been paid to their design and optimum utilization in order to achieve the most reliable and economical operation. This paper presents the results of performance optimization of a naturally ventilated PV/T system. A theoretical model is developed and validated against experimental data, where good agreement between the predicted results and measured data were achieved. The validated model was then used to study the effect of the channel length, channel depth, and channel width on thermal and electrical efficiency, PV cooling and outlet air temperature. Additionally, fast and elitist non-dominated sorting genetic algorithm (NSGA-II) method was applied to find the optimum values of geometric characteristics of the studied PV/T system for achieving higher thermal and electrical efficiency.