Ultrafast third-order optical nonlinearities of the as-deposited and annealed Au:Bi2O3 nanocomposite films de posited by magnetron cosputtering are investigated by using femtosecond time-resolved optical Kerr effect (OKE)and pump-probe techniques. The third-order optical nonlinear susceptibility is estimated to be 2.6 × 10-10 esu and 1.8 × 10-9 esu at wavelength of 800nm, for the as-deposited and the annealed film, respectively. The OKE signal of the as-deposited film is nearly temporally symmetrical with a peak centred at zero delay time, which indicates the dominant contribution from intraband transition of conduction electrons. For the annealed film, the existence of a decay process in OKE signal implies the important contribution of hot electrons. These characteristics are in agreement with the hot electron dynamics observed in pump-probe measurement.