The study was conducted to investigate the quality characteristics, bioactive compounds, and antioxidant activities of yogurt prepared by replacing powdered skim milk with 0%, 5%, 10%, and 15% sweet potato powder. Both pH and soluble solids decreased, but acidity increased in the control and treatment groups in proportion to the fermentation time of the yogurt and the sweet potato powder content. The lactic acid bacteria also increased rapidly as the fermentation time increased, and it was confirmed that the level of lactic acid bacteria in the experimental group in which 5∼15% of sweet potato powder was added was higher than that of the control group. The total polyphenol and flavonoid content of the yogurt increased in proportion to the amount of sweet potato powder added, and the antioxidant activity also tended to increase in proportion to the sweet potato powder content. The above results indicated that adding sweet potato powder to yogurt increases bioactive compounds, thereby enhancing antioxidant activity. It is suggested that adding sweet potato powder at levels of 5% to 15% by weight in place of skim milk powder can effectively utilize the functional properties of sweet potatoes in yogurt production.