Patients with gastroesophageal reflux disease (GERD) are often accompanied by otherfunctional gastrointestinal symptoms. In particular, there is a high percentage overlapwith the symptoms of irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO). The GERD patients have a significant higher prevalence of SIBO than control group. While, the SIBO patients have a significant higher prevalence GERD than control group also. An obvious feature of SIBO is that the hydrogen and carbon dioxide were produced by overgrown bacteria ferment carbohydrate in small intestine, and may produce methane in the presence of methanogens. Excessive gas can cause gastroesophageal reflux through three mechanisms: 1. abdominal distension and increased intra-abdominal pressure, compression of gastric contents retrograde into the esophagus; 2. part of the gas under the pressure upward through the stomach and esophagus; 3. relax esophageal sphincter by reflex. Understanding the relationship between GERD and SIBO is helpful to find out the underline cause of GERD and provide a new direction for the rational GERD treatment.