Two kinds of Ag2S nanoparticles with different surface-capping molecules (2,2’-bipyridine and hexametaphosphate ) were prepared by colloidal chemical method. Their third-order nonlinear optical properties have been studied in non-resonance region of one photon via degenerate four-wave mixing in phase conjugation geometry. The results show that two-photon resonant process greatly contributes to the third-order nonlinearities of Ag2S nanoparticles. It is also found that the third-order susceptibicities χ(3) are intensively subjected to the surface-capping molecules. This phenomenon is interpreted in terms of local field enhancement effect related to refractive index of the media around nanoparticles.