Sensitivity of pristine graphene (PG) and Pd-doped graphene (Pd-G) toward a series of small gas molecules (CO, NH3, O-2 and NO2) has been investigated by first-principles based on density functional theory (DFT). The most stable adsorption configuration, adsorption energy, charge transfer, density of states and magnetic moment of these molecules on PG and Pd-G are thoroughly discussed. It is found that four gas molecules are weakly adsorbed on PG with low adsorption energy of 0.08-0.24 eV, and the electronic properties of PG are only sensitive to the presence of O-2 and NO2 molecules. In contrast, doping graphene with Pd dopants significantly enhances the strength of interaction between adsorbed molecules and the modified substrate. The dramatically increased adsorption energy and charge transfer of these systems are expected to induce significant changes in the electrical conductivity of the Pd-G sheet. The results reveals that the sensitivity of graphene-based chemical gas sensors could be drastically improved by introducing the Pd dopants, so Pd-G is more suitable for gas molecules detection compared with PG. (C) 2015 Elsevier B.V. All rights reserved.