Human gut microbiota plays a key role in the development of obesity. Intestinal flora can regulate energy absorption and nutrition metabolism, increasing the energy harvesting from diet. Alteration of gut flora produces excessive lipopolysaccharide, which, when absorbed into the blood, can induce inflammatory reactions and promote the high-fat diet-associated obesity and metabolic syndrome. Intestinal flora increase visceral fat deposition by lowering the expression of Fiaf in intestinal mucosa. Different immune status also affects the intestinal flora.The gut microbiota is hypothesized to be an environmental factor that contributes to obesity; by interacting with factors such as host and diet, it adjusts the energy metabolism. Antibiotics or probiotics may alter the composition of intestinal microflora and improve the metabolic syndrome, and thus provides new treatment options.