Antioxidant activity and antimutagenic activities with and without simulated gastric digestion of hot water extracts from white and yellow onions were investigated as compared to BHT and ascorbic acid as control Contents of total phenol and flavonoid in hot water extract of yellow onion were higher than those of white one. The scavenging activity of hydrogen peroxide of both extracts were increased in direct proportion to added their concentration. Antioxidant activity and reducing power of the hot water extract were elevated through analysis of $eta-carotene-linoleate$ system and were lower than those of BHT and ascorbic acid. Antimutagenic activity after simulated gastric digestion of hot water extract of white and yellow onions was observed against mutagen IG and MNNG on Salmonella typhimurium TA80 and TA100. Extract of yellow onion was higher in antimutagenic activity than that of white one. In conclusion, these results suggested that phenol and flavonoid in hot water extract from yellow and white onions may play an important role in the antioxidant and antimutagenic activities.