The absorption spectra as well as the Hyper-Rayleigh scattering (HRS) signals of a highly monodisperse 10.5-nm diameter silver colloids with the addition of KNO3 and pyridine have been measured. Strongly enhanced HRS signals are observed by 15x and 6x, respectively. Through size distribution and TEM measurements. the morphology of aggregated silver nanoparticles has been observed as a chainlike aggregation. The dramatically enhanced second-order nonlinearity was explained by the enhanced electromagnetic (EM) field near the surface of the silver nanoparticles as they approached. The different enhancement between these two kinds of aggregates was considered with the diversity of separated distance between silver nanoparticles. The experimental results reveal that there is an optimum size for the aggregates to yield the maximum HRS response.