More and more attention has been paid to the aggregation behavior of nanoparticles,but little research has been done on the effect of particle size.Therefore,this study systematically evaluated the aggregation behavior of nano-silica particles with diameter 130-480 nm at different initial particle concentration,pH,ionic strength,and ionic valence of electrolytes.The modified Smoluchowski theory failed to describe the aggregation kinetics for nano-silica particles with diameters less than 190 nm.Besides,ionic strength,cation species and pH all affected fast aggregation rate coefficients of 130 nm nanoparticles.Through incorporating structural hydration force into the modified Smoluchowski theory,it is found that the reason for all the anomalous aggregation behavior was the different structural hydration layer thickness of nanoparticles with various sizes.The thickness decreased with increasing of particle size,and remained basically unchanged for particles larger than 190 nm.Only when the distance at primary minimum was twice the thickness of structural hydration layer,the structural hydration force dominated,leading to the higher stability of nanoparticles.This study clearly clarified the unique aggregation mechanism of nanoparticles with smaller size,which provided reference for predicting transport and fate of nanoparticles and could help facilitate the evaluation of their environment risks.