To upgrade methane to more valuable products, the non-oxidative coupling of methane (NOCM) is a hopeful method. This work investigates the mechanism and evaluates the feasibility of silica-supported single-site niobium (Nb) catalysts for NOCM by density functional theory (DFT) calculations. Two catalyst candidates were studied, namely, the silica-supported Nb(III)–methyl catalyst (denoted as (SiO)₂Nb–CH₃) and the silica-supported Nb(III)–hydride catalyst (denoted as (SiO)₂Nb–H). The catalytic cycle working with (SiO)₂Nb–CH₃ contains 5 elementary steps, and the one working with (SiO)₂Nb–H contains 6 elementary steps. (SiO)₂Nb–CH₃ is much more effective in catalyzing NOCM than (SiO)₂Nb–H on the basis of the energetic span value. Interestingly, the silica-supported Nb(III)–CH₃ catalyst can have an increase of 2 orders of magnitude in the turnover frequency (TOF) for NOCM compared to the silica-supported Ta(III)–CH₃ catalyst, the latter having been experimentally used for NOCM catalysis. This work provides further insights into the mechanism of single-site metal catalysts for NOCM, and provides a direct guide for catalyst design for NOCM as well.