Since the discovery of Ag-O-Cs photocathode, its mechanism of photoemission has remained enigmatic. The role of silver colloidal particles is important for the explana-tion of photoemission in long wavelength region. This paper presents a general review of the role of silver colloidal particles in the literatures, such as experiments by Лахомов et al. We come to a conclusion that the photoelectric emission of Ag-O-Cs photocathode above 4000? comes from small silver colloidal particles in the film of Cs2O.