Molecular simulation techniques are employed here to study the surface properties of nanocrystalline TiO 2 . Monte-Carlo method (1) is used to predict the properties of gas absorption on the surface of TiO 2 . The interaction energies between gas molecules and TiO 2 surface are calculated by the theory of open force field (2). The atomic model of TiO 2 is created by a 6 × 6 × 6 superlattice unit cell of rutile phase with a 15% facetting in {110} planes. CO, H 2 S, NO 2 gases are chosen as reacting gases in this study. The molecular simulations are conducted under the temperature of 473, 573, 673, 773, 873K and the pressure of 0.1, 0.5, 1.0 kPa. The results indicate that the interaction energies between gases and TiO 2 surface can be used to predict the selectivity and sensitivity of reacting gases in TiO 2 .