In the present study, we have compared the adsorption behavior of as-prepared and amine-functionalized MIL-100 and MIL-101 in the selective adsorption process. The synthesized adsorbents by the hydrothermal method were investigated by the different methods of analysis and identification. The volumetric method was used to investigate CO2 and CH4 adsorption at the different temperatures and P < 10 bar. Also, the selectivity of CO2 to CH4 was studied. The surface area and pore volume of MIL-101 were higher than those of MIL-100; consequently, the CO2 adsorption capacity of MIL-101 was higher than that of MIL-100. Although the surface area and pore volume of MIL-100 and MIL-101 significantly decreased after amine functionalization, the adsorbents showed the enhanced CO2 adsorption capacity at ambient temperature. The amine-modified adsorbents displayed ultrahigh selectivity for CO2 over CH4. Also, an increase in the temperature decreased the adsorption capacity of CO2. According to the results, modified MIL-101 can be a promising adsorbent for the purification and separation of gases.