The effect of tin on the structure, acidity, and water adsorption property of Nb2O5/α-Al2O3 catalyst and on this catalyst's performance for ethylene oxide (EO) hydration were studied by XRD, Raman spectroscopy, NH3-TPD, IR of pyridine and of H2O adsorption, and catalytic tests. Characterization results revealed that the density of acidic sites decreased with the addition of tin and with increasing Sn/Nb ratio in the cases studied, whereas the acidity exhibited minimum strength at Sn/Nb=0.08Sn/Nb=0.08. The IR of water adsorption indicated that addition of tin significantly enhanced the water adsorption property of the catalyst. The catalytic tests revealed that monoethylene glycol (MEG) selectivity increased from 89.8% of the Nb2O5/α-Al2O3 catalyst to 94.0% of the tin-promoted catalyst maintaining the EO conversion at 99.7%, and the catalyst exhibited excellent stability. The enhanced water adsorption property of the tin-promoted catalyst is proposed to be responsible for the significant improvement in MEG selectivity.