Three N-doped microporous activated carbon adsorbents were synthesized with KOH as the porogen agent at pyrolysis temperatures of 600, 700, and 800°C. The adsorbent samples were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, Raman spectra, Fourier transformed infrared (FT-IR) spectra, Energy-dispersive X-ray Spectroscopy (EDS) mapping, and volumetric nitrogen adsorption. CO2, CH4, and N2 adsorption isotherms were measured at 100 kPa and three different temperatures after activation. All isotherms were correlated with the Langmuir model. The heat of adsorption was calculated using the Clausius-Clapeyron equation based on single component adsorption isotherms. These data were used to estimate the separation selectivities for CO2/CH4, CO2/N2, and CH4/N2 binary mixtures at 298K and 100 kPa with the ideal adsorbed solution theory (IAST) model. Extremely good selectivites were achieved due to their N-containing groups, narrow pore size distribution, and large specific surface area. In addition, breakthrough simulations for CO2/CH4/N2 binary mixtures were conducted. The result is superior to ZIF-7 under the same condition. Based on these advantages, we infer N-ACs could be suitable and promising adsorbents for flue gas selective adsorption and natural gas upgrading.