This study evaluates changes in the chemical composition and bioactivities of American ginseng (Panax quinquefolius L.) processed by boiling in water, $75^{circ}C$ for 10, 20, 30, and 40 min, and autoclaving at high temperatures, $115^{circ}C$ for 30 and 60 min and $130^{circ}C$ for 90 and 120 min. Total ginsenoside contents of boiled ginseng remained relatively unchanged, whereas the contents of autoclaved ginseng samples significantly decreased with an increase of both time and temperature. Compared to unheated ginseng (control), the color of both boiled and autoclaved ginseng decreased in lightness and increased in redness. The acidic polysaccharide contents, the total phenolic contents and the antioxidant capacity of boiled and autoclaved ginseng were higher than the untreated ginseng, with the highest values being exhibited by the autoclaved samples. In particular, the antioxidant capacity of unheated ginseng increased about 2.5 times ($285.7{pm}14.03;mg$/100g to $777.2{pm}26.4;mg$/100g) when ginseng was autoclaved at $130^{circ}C$ for 120 min as compared to the control. It was concluded that as American ginseng was processed at a high temperature, especially steam-heated in an autoclave, its chemical constituents changed and, in particular, acidic polysaccharides, total phenolics and antioxidant capacity were considerably increased.