First-principles calculations within density functional theory have been carried out to investigate the adsorption of various gas molecules including CO, CO2, NH3, NO, NO2, CH4, H2O, N-2, O-2 and SO2 on MoS2 monolayer in order to fully exploit gas sensing capabilities of MoS2. By including van der Waals interactions between gas molecules and MoS2, we find that only NO and NO2 can bind strongly to MoS2 sheet compared to other gas molecules, in line with experimental observations. The charge transfer and variation of electronic structures are discussed in view of the density of states and molecular orbitals of gas molecules. (C) 2014 Elsevier B.V. All rights reserved.