Ever since the discovery of graphene, two-dimensional materials are promising to semiconductor industry. Monolayer molybdenum disulfide (MoS2) especially stands as a prospective candidate due to its stability in ambient environment; whereas, the direct bandgap makes it potential in electro-optical applications. Work function plays an important parameter for light-emitting diode and contact electrification [1-2]. Some studies [3-4] revealed magnetic properties of MoS2 by doping 3d transition metals, but the dependence of work function on adatoms remains vague. Under density functional theory (DFT) framework, the Perdew-Burke-Ernzerhof (PBE) exchange-correction functional is applied in Vienna ab initio Simulation Package (VASP). The cutoff kinetic-energy for the valence electron is 500 eV, where the convergence condition for the force acting on each atom is less than 0.01 eVÅ-1 and the energy difference is < 10-6 eV/cell. A 4×4 MoS2 supercell is modeled to simulate the adatom-MoS2 system, after the unit cell of MoS2 reaches equilibrium structure. The corresponding bandstructure and density of states are shown in Fig. 1, where the extracted bandgap of 1.71 eV agrees with the experiment . For the supercell, the Brillouin zone for structural optimization is sampled using 2×2×1 k-points by Monkhorst-Pack algorithm. The vacuum space of 15 Å is maintained to avoid the interaction from another layer of MoS2. The dipole correction on the z-direction is considered for the electrostatic potential away from the surface and total energy.