Using density functional theory (DFT) calculations, we explored the potential of defective MoS2 sheets decorated with a series of single transition metal (TM) atoms as electrocatalysts for N2 reduction reaction (NRR). The computed reaction free energy profiles reveal that the introduction of embedded single TM atoms significantly reduces the difficulty to break the N≡N triple bond, and thus facilitates the activation of inert nitrogen. Onset potential close to -0.6 V could be achieved by anchoring various transition metals, such as Sc, Ti, Cu, Hf, Pt, and Zr, and the formation of the second ammonia molecule limits the overall process. Ti-decorated nanosheet possesses the lowest free energy change of -0.63 eV for the potential determining step. To better predict the catalysis performance, we introduced a descriptor, φ, which is the product of the number of valence electron multiplying by the electronegativity of the decorated TM. It shows good linear relationship between the d-band center and the binding energy of nitrogen, except for those metals with less than half filled d-band. Although the metals in Group IIIB and IVB have strong adsorption interactions with N atom, the Gibbs free energy changes for desorption of the second ammonia are unexpectedly low. The selectivity of these systems towards nitrogen reduction reaction (NRR) are also significantly improved. Therefore, those defective MoS2 decorated with Sc, Ti, Zr and Hf are suggested as promising electrocatalysts for NRR, for their both high efficiency and selectivity.