Numerical study of transport phenomena in photo-electrochemical (PEC) reactor is developed. The present model comprises of the Navier-Stokes and the respective energy equations for electrolyte and the radiative transfer equation (RTE). The effects of various parameters on the hydrogen production rate and solar-hydrogen conversion efficiency are investigated. It is proposed to predict the reactor performance and optimize the objective function under various constraints, including operating conditions, for maximum of solar-hydrogen conversion efficiency and hydrogen production rate. Results have shown that the solar - to - hydrogen efficiency, η_SH is increased as solar flux increased. In addition, η_SH is increased as Energy band gap, Eg, decreased. This design can increase η_SH and the hydrogen volume production rate, Φ using anode is made of fluorine-doped tin oxide (FTO) coated with Fe2O3 semiconductor film.