Microbiota in human alimentary tract plays important roles for homeostatic maintenance of the body. Compositional difference of gut microbiota is tightly associated with susceptibility of many diseases,including inflammatory diseases,obesity,diabetes mellitus,cancer,and atherosclerosis. “Dysbiosis” refers to a state of imbalance among the colonies of microorganisms within the body,which brings abnormal increase of specific minor components and decrease in the normally dominant species. Since stomach secrets strong acid for its digestive role,this organ has long been thought a sterile organ. However,the discovery of Helicobacter pylori(H. pylori) has changed the concept. This bacterium has proven to cause gastritis,peptic ulcer,and gastric cancer. However,recent cross-sectional studies revealed that H. pylori carriers had a decreased risk of developing immunological diseases,such as asthma. H. pylori coinfection also suppresses inflammatory bowel diseases. This review describes human gastric microbiota by discussing its mutual interaction and pathogenic enrollment. Gastric “dysbiosis” may affect host inflammatory response and play important role for gastric pathogenesis. We will topically discuss enrollment of dysbiosis for genesis of gastric cancer as well as for disruption of immunological homeostasis affecting oncogenic resistance. Microbiota in human alimentary tract plays important roles for homeostatic maintenance of the body. Compositional difference of gut microbiota is tightly associated with susceptibility of many diseases, including inflammatory diseases, obesity, diabetes mellitus, cancer, and atherosclerosis. “Dysbiosis ” refers to a state of imbalance among the colonies of microorganisms within the body, which brings abnormal increase of specific minor components and decrease in the normally dominant species. this body has long been thought a sterile organ. This discovery of Helicobacter pylori (H. pylori) has changed the concept. This bacterium has proven to cause gastritis, peptic ulcer, and gastric cancer. However, recent cross-sectional studies revealed that H. pylori carriers had a decreased risk of developing immunological diseases, such as asthma. H. pylori coinfection also suppresses inflammatory bowel diseases. This review describes human gastric microbiota by discussing its mutual interaction and pathogenic enrollment. Gastric “dysbiosis ” may affect host inflammatory response and play important role for gastric pathogenesis. We will top discussively enrollment of dysbiosis for genesis of gastric cancer as well as for disruption of immunological homeostasis affecting oncogenic resistance.