Design of novel catalysts for the reduction of N to ammonia has been urgently pursued because of various issues related to the industrial reduction technology. In this work, we perform first-principles calculations on the basis of the density-functional theory to control the edges of two-dimensional (2D) transition-metal disulfides (TMDs), including MoS, WS, VS, NbS, TiS, and TaS, for the achievement of optimal efficiency in nitrogen-fixation. Our calculations show that nitrogen molecules prefer to stay at the bridge-on sites of the metal edges of TMD nanoribbons because of exothermic reactions. The calculated energy barrier at each step illustrates that VS has the lowest potential-determining step of 0.16 eV in the distal pathway, leading to its best catalytic activity in the N reduction reaction (NRR). Additionally, we find that the trend of catalytic activity of 2D TMD nanoribbons is as follows: VS > NbS > TiS > MoS > WS > TaS. We show that charge transfer is critical to the reduction reaction. We further demonstrate that the edges of TMDs, especially VS, show a higher selectivity for NRR over the hydrogen evolution reaction (HER) by investigating the competition between HER and NRR. Our findings not only reveal the effect of the edges of TMDs on NRR, but also provide theoretical support to the reported experimental results in the literature. It is expectable that the 2D TMD nanoribbons, especially VS, may find application for efficient N-fixation. At the same time, our work may guide the design of new catalysts for NRR.