BACKGROUND/AIMS: This study assessed the effects of the raw and heated garlic on Helicobacter pylori (H. pylori) infection. METHODS: The concentration of garlic thiosulfinates was determined by spectrophotometric method from the raw and heated aqueous garlic extract. Bacterial suspensions of H. pylori, Escherichia coli, Staphylococcus aureus, and Candida albicans were inoculated into their culture media containing thiosulfinates at concentrations of 0, 20, 40, 80, and 160 microgram/mL, respectively. Viability was measured by the plate colony count technique. In vivo effects of raw and heated garlic on H. pylori-infected gerbil model were also determined. RESULTS: Raw garlic showed a strong activity at low concentrations of thiosulfinates against H. pylori and other microorganisms, compared with weak activity of heated garlic. Chronic and active gastric inflammation were significantly improved by the treatment of 0.8 mg/mL raw garlic compared to H. pylori-infected control group. However, such effect was not observed at 1.6 mg/mL dose. Heated garlic had no beneficial effect. CONCLUSIONS: Raw garlic has an excellent inhibitory activity against H. pylori infection.