Chitooligosaccharide (COS), which is one type of the oligosaccharides, is produced from chitin or chitosan by chemical or enzymatic decomposition methods. Some studies in fish regard that adding COS in dietary could produce beneficial effects on growth performance, nonspecific immune, or blood profiles. about which was effected by COS on promoting growth performance and intestinal microorganisms and =450, average initial body weight=3.02±0.16 g) into 5 groups consisting of triplicate (=30 fish/replicate). Each group was hand-fed a diet containing either 0 (control group), 0.10%, 0.30%, 0.50% or 0.70% COS to satiation three times daily for 8 weeks, and fish were reared in 400 L aquaria with the temperature of (27±2intestinal microorganisms, nutrient16.17% and 9.47%All groups with COS significant increases in specific growth rate (SGR) and the digestibilities of dry matter and protein whereas feed conversion ratedecreased significantly Compared with control group, intestinal villus width increased by 45.21% and 54.06% ( Lacto bacillus of juvenile tilapia (0.05). Results above showed that additional COS improved the intestinal environment and the growth performance, therefore the appropriate addition is 0.30%~0.50% diet of juvenile GIFT tilapia.