Abstract : Polysaccharides of marine algae belong to anion groups such as fucoidan and alginate are known the treating ability of gastric ulcers but carrageenan has not seen the announcement yet. Therefore, the paper focused on the treating ability evaluation of gastric ulcers of carrageenan that was extracted from Kappaphycus alverazii grown in Vietnam. Three acid agent (hydrochloric acid, 5-sulfosalicylic acid, and acetic acid) in various concentration (1, 5, and 10%) was used for causing disease. Various concentration of carrageenan (0.5, 1.0, and 1.5%) was used for the treating of gastric ulcers. Guinea pig (Cavia porcellus) was observed clinical pharmacology (rate of blood sedimentation and gastric disease surgery) on day 5th and 10th after the treatment. The results showed that carrageenan has the treatment effect of gastric ulcers. A concentration of 1.5% (w/v) carrageenan is a suitable choice for the protection of the gastric mucosa of mice that drunk 10% hydrochloric acid after a 10-day treatment. In summary, carrageen extracted from Kappaphycus alverazii grown in Vietnam can completely be used in food and pharmaceutical products, especially in the preparation of drugs, and functional foods to support the treatment of gastric ulcers.