Abstract: The action of rare earth elements doped in Ag-BaO thin films is discussed, based on the colloidal particles theory and experimental results. It shows that, influenced by the size and shape of Ag nanoparticles, increasing optical absorption and improving transport condition are two ways to enhance the photoemissive current density. With the action of rare earth elements, the Ag particles become smaller and spheroid. As a result, the total surface square of Ag particles increases, the optical absorption increases, the equivalent barrier between a metallic particle and the host semiconductor decreases, and the transport condition improves. So, the rare earth elements can enhance the photoemission of Ag-BaO thin film.