The ultraviolet visible photoabsorption spectrum of the Ag particles matrix (BaO) thin film deposited by vacuum evaporation was measured. The transmission electron micrograph indicated that in the matrix Ag formed particles with a mean diameter of 15 nm. There is a clear absorption peak of surface plasmon mode at 410 nm wave length. This is the absorption of electric dipole oscillation. The results were compared with those in previous literatures. The dependence of absorption on the particle size was discussed.