The 2nd landfill of Sudokwon landfill site has been collecting landfill gas flowing into vertical wells using the vacuum generated by blowers. The vacuum applied to the wells enables the landfill gas within the collection boundary to be collected and the air in the atmosphere to be penetrated through landfill cover soil, as well. In this study, we tried to investigate the impact of active landfill gas collection with vertical wells on landfill aeration. The pressure changes and the compositions of landfill gas, measured at the wells and 4 bore holes made in the landfill for this study, showed the air penetrates the landfill cover and inflows into the vertical wells. The concentration of nitrogen in the landfill gas collected from the 2nd landfill site has been increased to 14 ~ 15% as of February 2013, indicating 18 ~ 19% of the air is being included in the landfill gas.