Aqueous sols containing nanocrystalline SnO2 particles with sizes less than 2 nm were prepared by ultra-filtration from original acidic and alkaline sols with SnO2 particles. The sizes of SnO2 nanocrystallines were determined by dynamic light scattering, X-ray diffraction, and computer analysis of nanocrystalline images of transmission electron microscopy. Blue shifts of absorption edges of the nanocrystallines were observed in both the ultra-filtrated acidic and alkaline sols. The results show that the blue shifts of the SnO2 nanocrystallines are mainly attributable to the size decrease for the same kind of nanocrystallines.