Nitrogen is a serious contaminant in natural gas because it decreases the energy density. The natural gas specification in South Korea requires a N2 content of less than 1 mol%. Thus, cost-effective N2 removal technology from natural gas is necessary, but until now the only option has been energy-intensive processes, e.g., cryogenic distillation. Using porous materials for the removal process would be beneficial for an efficient separation of CH4/N2 mixtures, but this still remains one of the challenges in modern separation technology due to the very similar size of the components. Among various porous materials, metal-organic frameworks (MOFs) present a promising candidate for the potential CH4/N2 separation material due to their unique structural flexibility. A MIL-53(Al), the most well-known flexible metal-organic framework, creates dynamic changes with closed pore (cp) transitions to open pores (ops), also called the ‘breathing’ phenomenon. We demonstrate the separation performance of CH4/N2 mixtures of MIL-53(Al) and its derivative MIL-53-NH2. The CH4/N2 selectivity of MIL- 53-NH2 is higher than pristine MIL-53(Al), suggesting a stronger CH4 interaction with NH2.