The antioxidant properties of red ginseng extracts prepared under different extraction conditions were evaluated by a variety of antioxidant assays, including $DPPH^{cdot}$ radical scavenging, $ABTS^{+cdot}$ radical scavenging, superoxide anion scavenging, nitrite scavenging and reducing power activities. The contents of total phenolic compounds and flavonoids were also determined. The various antioxidant activities were compared to positive controls such as Trolox, tannic acid and ascorbic acid. The antioxidant activities of all of the extracts were shown to be the highest in the ethanol extract. The antioxidant activities of the red ginseng powder were the lowest among the samples. The amounts of total phenolic compounds and flavonoids were at a maximum in the ethanol extract. Correlation analysis demonstrated the existence of a linear relationship between free radical scavenging activities and the phenolic compounds contents of extracts. The antioxidant activity of yogurt was increased as the result of the addition of red ginseng extract. The quality characteristics of the yogurt to which red ginseng extract was added were similar to those of yogurt without red ginseng extract. The overall sensory score and color of yogurt made from 0.5% red ginseng was the best of the tested yogurts. In accordance with the antioxidant activity and quality characteristics, the optimal concentration of red ginseng extract was approximately 0.5%.