In this paper, the rule of photoemission from the small silver colloidal particles in the film of Cs2O is discussed, and the formulas of energy distribution of photoelectrons, photoelectric current and quantum yield are derived. The theoretical curves of energy distribution of photoelectrons, long wavelength response and quantum yield of Ag-O-Cs photocathode are compared with experimental results. It is found that for the long wavelength response the equivalent diameters of small colloidal particles are about 31?.The problems of the treatment of Cs, the additional silvering process and the ques-tion about Fowler curve are also discussed and explained.