SnO2 nanoparticles were prepared using the hydrothermal method. The particle size was gradually increased with increasing the hydrothermal temperature and period. The size of SnO2 nanoparticles was successfully controlled in the range of 2.3-5.9 nm. The deposition of SnO2 nanoparticles on to Al2O3 was made through impregnation process. TEM and XRD measurements revealed that SnO2 nanoparticles are highly dispersed on the surface of Al2O3 without changing the particle size. When SnO2 nanoparticles prepared at 150 A degrees C for 3 h were deposited on to Al2O3 (denoted as SnO2-150-3/Al2O3), a part of SnO2 nanoparticles was suspected to be dispersed inside of the pores of Al2O3. This is probably due to a small size of SnO2 nanoparticles compared with the pore diameter of Al2O3. On the other hand, SnO2 nanoparticles with relatively large size were found to be predominantly present on the outer surface of Al2O3. H-2-TPR measurements suggest the presence of SnO2-Al2O3 interaction in SnO2/Al2O3 catalysts. Especially, SnO2-150-3/Al2O3 was considered to include SnO2 nanoparticles interacting more strongly with Al2O3. SnO2/Al2O3 catalysts showed higher catalytic activity for the selective reduction of NO with C3H6 than Al2O3, suggesting that the SnO2/Al2O3 prepared using SnO2 nanoparticles is effective to develop highly active catalyst.