Since in-feed antibiotics (IFAs) are being taken out of broiler diets around the world, beginning in Sweden in the year 1986, the search for alternatives to replace IFAs has gained increasing interest in animal nutrition. Gut microflora appears to be the target for IFAs and alternatives to exert health benefits and some growth-promoting effects. In this review the effect of six kinds of alternatives to IFAs on gut microflora is discussed and their working mechanisms and growth-promoting effects are reviewed. This review focuses on mannanoligosaccharides (MOS) as alternative to antibiotics using research results obtained by various authors in recent years.