In this study, we evaluated the gastric protective activities of mokdanpi in vitro. Further, we used experimental ulcer models to identify the active ingredients of mokdanpi. As a preliminary evaluation of mokdanpi ethanolic extract and its ingredients, we assessed its radical scavenging activities. In addition, its antimicrobial activity against Helicobacter pylori (H. pylori) was investigated. The antiulcerogenic activity of the active ingredients was evaluated in pylorus-ligated rats, an HCl/ethanol-induced and an absolute ethanol-induced ulcer model. We confirmed the scavenging effect of the ethanolic extract of mokdanpi and its ingredients against 2,2-diphenyl-1-picrylhydrazyl, nitric oxide and superoxide radicals, and we demonstrated that mokdanpi could inhibit the colonization of H. pylori. In an HCl-ethanol-induced ulcer model, gallic acid and catechin (100 mg/kg) inhibited 40.6% and 41.7% of gastric lesions, respectively. Catechin (100 mg/kg) significantly reduced (p<0.05) the gastric secretion induced by pylorus ligature in rats in comparison to the control group. Gallic acid (100 mg/kg) significantly increased (p<0.05) the mucus contents in an ethanol-induced ulcer model. The antioxidant ingredients (catechin and gallic acid) present in mokdanpi play a major role in antiulcerogenic activity, and demonstrate novel activity against H. pylori.