We have developed a new production method for silica sols with a particle size of 3-5 nm using the Y-shaped reactor. Diluted sodium silicate and sulfuric acid were forced to collide with each other at the intersection of the reactor and neutralize at the exit. Silica sols with the appropriate viscosity produced by this method could be used as waste water treatment agents and a retention aids for papermaking. The gel aging phenomena of silica sol were considered in terms of their viscosity and electric conductivity. The objective of this study is to find suitable operation conditions for silica sols and to clarify mechanisms of the aging. [DOI: 10.1380/ejssnt.2009.737]