The influence of oxygen partial pressure on the Fermi level of ZnO films prepared by pulsed laser deposition is investigated.The contact potential difference of the ZnO films fabricated under various oxygen partial pressures is studied systematically using Kelvin probe force microscopy.The Fermi level shifted by 0.35eV as oxygen partial pressure increased.This indicates a significant change in the electronic structure and energy balance in ZnO films.This fact provides a consistent explanation that the changes in carrier concentration,resistivity and mobility of ZnO films are attributed to oxygen vacancy induced shift of the Fermi level.