The second-order optical nonlinearity of US nanoparticles with different mean diameters of 28.0, 30.0, 31.5, 50.0, and 91.0 Angstrom was studied by the incoherent hyper-Rayleigh scattering technique. Results show that the first-order hyperpolarizability value per US particle decreases with decreasing size from 91.0 to 31.5 Angstrom; however, as US particle size further decreases, this trend is reversed and the beta value increases. Substantially, the normalized value of the first-order hyperpolarizability per US formula unit, beta(0), exhibits systematic enhancement with decreasing size. This is interpreted in terms of a so-called surface contribution mechanism. The two aspects of quantum size effects, the size dependence of optical band-gap and oscillator strength, cannot be adopted to explain the size dependence of the second-order optical nonlinearity of US nanoparticles. (C) 2003 Elsevier Inc. All rights reserved.