Activated carbon was oxidized by an acid mixture of concentrated HNO₃ and H₂SO₄ with a volume ratio of 1∶1 diluted with the same volume of de-ionized water, followed by reduction with iron dust to modify its surface chemical properties for CO₂ adsorption. The samples before and after the modification were characterized by N₂ adsorption, elemental analysis, FT-IR and XPS. CO2 adsorption properties of the samples were investigated by high pressure adsorption. It was found that the adsorption capacities of the modified sample were 17.72 and 14.01mmol/g at room temperature and 319.15K respectively, showing an increase of 49 and 70% respectively, based on the surface area of the samples. The modified activated carbon can be recycled 4 times at ambient conditions without a significant decrease in CO₂ adsorption capacity. Surface basic nitrogen-containing functional groups on the modified sample, such as primary amino groups, are responsible for the increased adsorption.