The subpicosecond optical non-linearities of PbS nanoparticles modified by cetyltrimethylammonium bromide (CTAB) and sodium dodecyle benzene sulphonate (DBS) was synthesized and investigated by femtosecond optical Kerr effect (OKE) experiment. The non-linear optical effect (chi((3)/)alpha for PbS(CTAB) and PbS(DBS), 2.6 x 10(-13) and 8.4 x 10(-14) esu cm, respectively) with the response time comparable to the laser pulser (similar to 165 fs) had been observed. The spectral behavior and the OKE results confirmed that the surface environment plays a crucial role on linear and non-linear optical properties.