The outstanding of fructooligosaccharide as a nondigestible dietary fiber has been proved for the prevention of hypercholesterolemia, overgrowth of pathogen bacteria, colon cancer, and enhancement of mucosal immune response.The primary function of beneficial intestinal microflora is to protect the intestinal tract from the proliferation of infection or harmful bacteria. Bifidobacterium bifidum produces volatile fatty acids, which provide important metabolic energy, and acidifies the bowel, which inhibits the growth of harmful bacteria, including Salmonella, Shigella, Clostridium, Campilobacter jejuni, and Escherichia coli. When the population of beneficial microflora decreases the gastrointestinal environment makes a subtle chemical shift, which allows harmful bacteria such as Clostridium perfringens and E. coli to proliferate. One clinical manifestation of this imbalance is diarrhea. In developing countries, diarrhea is still the most important cause of child morbidity and mortality with an estimation of 1.3 billion episodes each year in children under 5 years old. FOS acts like an efficient intestinal “fertilizer” by feeding beneficial intestinal microflora and helping them to reproduce. The improvement in the intestinal microflora was followed by a relief of constipation, or loose stool;decreased formation of putrefactive products in the large intestine, improved serum lipids in hyperlipidemia; and reduced total cholesterol, triglycerides, blood glucose, and blood pressure. Studies show that the duration of diarrhea in children who ingested FOS were shorter than in those who ingested a placebos (2.62 days versus 4.24 days). The pH of stool in children who ingested FOS was significantly lower than in children who did not.