Antioxidant activity and flavonoid content of Welsh onion (Allium fistulosum) (green-leafy and white-sheath varieties) and the effect of thermal treatment on them were studied by comparing with those of onion (Allium cepa) (yellow and red varieties). Antioxidant activity was measured by Trolox equivalent antioxidant capacity (TEAC) and ferric reducing antioxidant power (FRAP) assays. The order of these indices of antioxidant activity was red onion>yellow onion=green Welsh onion>>white Welsh onion. The order of the total flavonoid content was red onion>yellow onion>green Welsh onion>>white Welsh onion. Major flavonoid of yellow and red onions was quercetin, and that of green Welsh onion was kaempferol. Antioxidant activity of green Welsh onion was increased, but that of the other three vegetables was decreased during boiling for more than 15 minutes. Flavonoids in green Welsh onion were less stable than those in the other three vegetables during the boiling procedure. These results suggested that green Welsh onion, but not the white one, is a potent antioxidant food comparable to yellow onion, and is a good source of kaempferol. Increased antioxidant activity and decreased flavonoid content during boiling were characteristics of green Welsh onion.