The adsorption of pure gases of carbon dioxide, methane and nitrogen was examined on nitrogen-doped porous carbon (NDC) prepared from polyaniline (PANI) as precursor by chemical activation with KOH (T=650-°, IR=2, t=1 h) to determine the potential for the separation of CO2 gas from flue gas or natural gas. Adsorption equilibrium of all gases was determined in a temperature range of 298-318 K and pressure up to 14 bar. Results demonstrated an excellent CO2 adsorption capacity of 3.09mmoㆍg-1 owing to high CO2 interaction affinity with NDC surface compared to CH4 (1.43mmolㆍg-1) and N2 (0.64mmolㆍg-1) under ambient condition (298 K and 1 bar). The ideal adsorbed solution theory (IAST) was used to determine the adsorption selectivity of NDC for CO2/CH4 and CO2/N2 mixtures at different compositions. The NDC had CO2/CH4 (4.42 and 4.08 for CO2/CH4=10/90 and 50/50, respectively) and CO2/ N2 (12.81 and 12.08 for CO2/N2=15/85 and 50/50, respectively) IAST selectivity at temperature of 298 K and pressure of 1bar. The moderate CO2 adsorption enthalpy indicates that N-doped activated carbon is a promising material in gas separation such as natural gas and flue gas processing.