The pump-probe technique was used for measuring the transient optical relaxation of Cu-BaO composite thin film which was stimulated by femtosecond laser pulses. The experimental results were obtained that the optical transmissivity of the thin film rapidly decreased and a negative peak emerged during the time delay. The relaxation process consisted of two components: a fast recovery with a time of 500 fs and a subsequent slow one with over several picoseconds. An amended theoretical value of the electron-phonon coupling constant g of Cu nanometer particles in the composite thin film is given as 1.3 x 10(17) W/m(3) K according to the experimental results. The optical relaxation is a process in which nonequilibrium electrons. originating from Cu nanometer particles and excited by ultrashort laser pulses, return to the equilibrium state.