Adsorption of N2 and CH4 in single-walled carbon nanotubes (SWCNTs) was simulated using grand canonical Monte Carlo simulations[1][2]. Firstly, the adsorption behaviors of pure N2 and CH4 in SWCNTs with different pore sizes at 300 K under various pressures were investigated. The adsorption capacities of pure N2 and CH4 both show an increasing trend with the increasing simulated pressure and SWCNT pore diameter. At the same simulated pressure and adsorbent, the adsorption capacity of CH4 is consistently higher than that of N2. And this difference between them becomes increasingly significant as the adsorbent pore size enlarges. Secondly, the effect of SWCNTs pore size on CH4/N2 adsorption selectivity is studied accordingly. When the simulated gas is an equimolar mixture of N2 and CH4, the adsorption capacities of both components also show an increasing trend with rising simulated pressure and increasing SWCNT pore size. Nevertheless, it can still be observed that, at the same temperature, pressure, and adsorbent, the adsorption amount of CH4 in SWCNTs is consistently higher than that of N2. SWCNTs show higher adsorption selectivity toward CH4, which is enhanced as the pore size decreases. Energetically, CH4 exhibits a higher adsorption heat and has stronger interactions with SWCNTs, both of which increase with decreasing pore size. Entropically, the marginal advantage of N2 is insufficient to compensate for its energetic deficiency relative to CH4. Moreover, CH4 occupies a larger filling space and achieves higher packing efficiency, further diminishing the advantage of N2 on the level of the entropy of adsorption.