Poly-crystal LiNbO3 Films on a fused SiO2 substrate are prepared by the rf sputtering technique. The third-order optical nonlinearity of the LiNbO3 fiIms has been investigated by using the Z-scan method with picosecond laser pulses at the wavelength of 532nm. The nonlinear refractive index γ and nonlinear absorption coefficient β are determined to be -7.2 x 10-9 cm2/W and 0.39cma/kW, respectively. The third-order optical nonlinearity X(3)of the LiNbO3 films is about 9.1 × 10-7esu, which is considerably larger than the reported value for Z-cut pure LiNbO3 crystals.