Adsorption of small gas molecules (O-2, NO, NO2 and NH3) on transition-metal Cu atom embedded mono layer MoS2 was investigated by first-principles calculations based on the density-functional theory (DFT). The embedded Cu atom is strongly constrained on the sulfur vacancy of monolayer MoS2 with a high diffusion barrier. The stable adsorption geometry, charge transfer and electronic structures of these gas molecules on monolayer MoS2 embedded with transition-metal Cu atom are discussed in detail. It is found that the monolayer MoS2 with embedded Cu atom can effectively capture these gas molecules with high adsorption energy. The NH3 molecule acts as electron donor after adsorption, which is different from the other gas molecules (O-2, NO, and NO2). The results suggest that MoS2-Cu system may be promising for future applications in gas molecules sensing and catalysis, which is similar to those of the transition-metal embedded graphene. (C) 2016 Elsevier B.V. All rights reserved.