The first-order hyperpolarizability beta values of two kinds of CdS nanoparticles with different surfaces were experimentally determined in solution via an incoherent hyper-Rayleigh scattering (HRS) technique. It is found that the P value of CdS nanoparticles capped with 2,2'-bipyridine is 4.27 x 10(-27) esu which is about a factor of 3 smaller than the beta value (1.30 x 10(-26) eSu) Of bare CdS nanoparticles whose surface is not modified by organic groups. This difference is interpreted in terms of a two-level model approximation derived from molecular chromophores, when considering the contribution of moleculelike scatterers at the particle surfaces.