Abstract: According to that the blood group substance in saliva can specifically combine to it's corresponding antibody, the method of neutralization test was adopted to determine the differences in the concentration of the blood groop substance between 60 cases of healthy people whose blood types were known and 30 cases of patients with carcinoma of stomach, as well as the differences after and before operation in patients with gastric cancer, and also between patients with poorly differentiated gastric adenocarcinoma and patients with other pathologic types. The result was statistically analysised. The concentration of A type and H type blood group substance in patients with gastric cancer elevated significantly as contrast to those in healthy people (A type, P <0.01; O type, P <0.05). The concentration of blood group substance in patients of A blood type but not in patients of B or O blood type decreased obviously. The result indicates that concentrationes of A and H blood group substance in patients with poorly differentiated adenocarcinoma elevated significantly as compare to those in patients with the other pathologic types (P <0.01). All the results suggest that determination of the blood group substance can be used as an auxiliary index of diagnosing the gastric cancer and also as a prognostic indicator for the patients of A and O blood type.