By using degeneratefour-wave mixing, the third-order nonlinear optical properties of nc-Si:H thin films were investigated.The phase conjugation signal was observed, and the third-order nonlinear susceptibilitiesat 589 nm for the volume fraction XC1=15% and XC2=30% of thecrystalline in the samples are χ1(3)=3.8×10-6 esu and χ2(3)=4.3×10-7esu, respectively. The nonlinear machanism of the material was discussed.