The hyper-Rayleigh scattering(HRS) technique has been employed to determine the second-order nonlinear optical response of a highly monodisperse silver colloid and its aggregation which is induced in the presence of KNO3. Through the size distribution and TEM measurements, the silver nanoparticle aggregates have been observed as a chain-like aggregation. The experimental result reveals that HRS signal increases 15 times when the aggregation average size is up to 120nm.