Active catalysts for nitrogen fixation (N-fixation) have been widely pursued through constant efforts for industrial applications. Here, we report a family of catalysts, MXenes (MX: M = Mo, Ta, Ti, and W; X = C and N), for application in N-fixation based on density functional theory calculations. We find that the catalytic performance of MXenes strongly depends on the reaction energy in each reaction step. More exothermic steps lead to higher catalytic performance in the course of N-fixation. We show that the reaction energy in N-fixation is strongly affected by the charge transfer: (1) if N atoms gain more electrons in a step, the reaction is exothermic with a larger reaction energy; (2) if N atoms lose electrons in a step, the reaction is endothermic in general. We further show that MoC and WC are highly active for N-fixation due to their exothermic reactions and strong charge transfer, which may be applicable in the chemical-engineering industry.