The pump-probe method was used for measuring the transient change of optical transmissivity of a composite thin film Au-Ba-O which was stimulated by a femtosecond pulsed laser. A phenomenon was observed that the optical absorption in the thin film rapidly increased and then recovered in a few picoseconds. Optical relaxation is a process in which nonequilibrium electrons, excited by laser pulses and originating from Au ultrafine particles, return to the equilibrium state. In this paper, a theoretical value of the electron-phonon coupling constant g of Au ultrafine particles in the composite thin filim was given.