The third-order nonlinear optical susceptibility chi((3)) of nanoscale Ag particles dispersed in silicon oil and surface-modified nanoscale Ag particles dispersed in silicon oil was measured by a third harmonic generation (THG) method. Experimental results show that the third-order nonlinear susceptibility chi((3)) was about 9.8 X 10(-11) esu and 7.0 X 10(-12) esu for the nanoscale Ag particles and the nanoscale Ag particles modified by dressing agent, respectively, at the incident light wavelength of 1064 nm. It is suggested that local field effect and the interband transition of electrons from the surface state to the unoccupied state near the Fermi level played important roles in the enhancement of chi((3)) for the nanoscale Ag particles. The interaction between the dressing agent and the surface of nanoscale Ag particles seems to have a strong influence on third-order nonlinear optical properties.