The functional properties of Korean red ginseng (KRG) are known to be dependent upon the extent of heat processing. We demonstrated that repeated heat processing in the preparation of KRG decreased its arginyl-fructosyl-glucose (AFG) content and metal chelating capacity. On the other hand, the total phenolic compound content, in vitro peroxyl radical scavenging ability, and cellular peroxyl and hydroxyl radical scavenging ability increased with repeated heat processing. The peroxyl radical scavenging ability of KRG may be due to the total phenolic compound content. The hydroxyl radical scavenging ability of KRG seems to result mainly from the chelating ability of Amadori compounds such as arginyl-fructose and AFG. The protective ability of KRG against Cu²(+)-induced oxidative stress in hepatoma HepG2 cells may be due to permeability into the cell membrane rather than antioxidant activity. These results can be used to predict the physiological functions of KRG in in vivo models.