Sweet potato leaves, a by-product of sweet potato production, are mainly discarded in Japan, although their rich polyphenol content gives them potential utility as animal feed. Since their use in animal feed requires several months' preservation, research has focused on the production of dried leaves. Drying leaves with an airflow dryer at selected drying air temperatures (60.0 degrees C, 80.0 degrees C, 100.0 degrees C, 120.0 degrees C, and 150.0 degrees C) resulted in substantial degradation of leaf polyphetzol content. The leaf polyphenol content after drying remained no more than 19.2% of that of raw leaves, regardless of the drying air temperature. The aim of this study was to develop a drying method that preserves leaf polyphenol content. Variations in total polyphenol content and leaf temperature during drying were investigated; polyphenol content decreased rapidly when leaf temperature was greater than 36.0 degrees C. The polyphenol content of dried leaves was improved to 44.1% of that of raw leaves when the drying air temperature was adjusted to maintain leaf temperature below 36.0 degrees C. When a 6 min steam blanching was applied prior to drying, at least 76.8% of the initial polyphenol content remained after drying.