The electron work function (EWF) is the minimum energy required to move an electron at the Fermi level from inside a conducting material to its surface with zero kinetic energy. This fundamental parameter is directly related to many chemical, physical, and mechanical properties of materials. In this work, variations in EWF of TiO(2) nanotube arrays under light illumination were monitored in situ using a Kelvin probe in order to study the photon-induced electron excitation in the TiO(2) nanotubular arrays upon illumination. It was shown that the EWF could be used to investigate the electron-hole separation and recombination, helping us to better understand the photo-activity of the photocatalytic material. This study has demonstrated that EWF provides an effective parameter for understanding of semiconductors' photo-activities with different views that may not be achieved using traditional techniques, such as diffuse reflection spectroscopy and photoelectrochemical measurement.