Chenopodium album Linne (CAL) is a fast-growing weedy annual plant. The leaves and young shoots may be eaten as a leafy vegetable. In oriental medicine, CAL has been used for treatment of skin disease, fever, stomach ache, toothache, and paralysis. After a preliminary screening of CAL ethanol extract and its fractions obtained from CAL leaves for anti-gastritic and anti-Helicobacter pylori (H. pylori) activity, the butanol (BuOH) fraction was found to have the most significant effect. We also examined antioxidative properties of the total CAL extract and its fractions, and also betaine as an ingredient of the BuOH fraction. To investigate the antioxidant effects of CAL on gastritis, the reducing power, free radical scavenging activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH), and lipid peroxidation effects were determined. Additionally, the BuOH fraction reduced cell viability in a concentration dependent manner in human gastric cancer cell lines. The results of this study revealed that CAL has excellent antioxidant activity, and may be useful in treating gastritis and gastric cancer.