Drying is one of the most important steps in preserving bioactive compounds or other ingredients in onions. This study evaluated the moisture removal and physicochemical quality of red onion slices with various air temperatures. The harvested onion with a moisture content of around 82.6% was sliced about 4 and 6 mm thick and dried in the convective dryer from 40°C to 120°C. The moisture content, rehydration ratio, color, quercetin content, and antioxidant activity were evaluated. Results showed that the moisture removal from onion at the higher temperature was faster, and the onion still had the antioxidant ability. However, at operational temperatures upper 80°C, the structure of onion tissue and its organic substances deteriorated, which degraded the appearance, rehydration ratio, and quercetin content. Therefore, the drying temperature of 80°C or below is recommended to retain the physicochemical quality of onion.