Abstract: This experiment was conducted to investigate the effects of Enterococcus faecalis on growth performance, body composition, digestive ability and blood physiological and biochemical parameters of GIFT tilapia (Oreochromis niloticus). 300 fish of initial body weight (50.59±0.59)g were randomly divided into 5 groups, and each group was fed to triplicate repeats of 20 fish for 60 days. Each group was fed five isonitrogenous (crude protein 36%) and isolipidic (ether extract 6.75%) diets which added respectively 1.3×10²(control), 1.4×10⁵, 1.7×10⁶, 1.5×10⁷and 1.8×10⁸ CFU/g (measured) E. faecalis. The results showed as follows: ① Compared with control group, the final body weight (FBW), weight gain rate (WGR), specific growth rate (SGR) and feed intake (FI) in the group of 1.5×10⁷ CFU/g reached a maximum and were significantly increased, and the feed conversion ratio (FCR) in this group was significantly reduced. Based on FBW, WGR and SGR, quadratic regression analysis projected the optimal concentration range for GIFT tilapia feed to 7.5×10⁷–1.1×10⁸ CFU/g. ②Compared with control group, the whole body crude protein content in all addition groups was significantly increased, and the whole body crude fat content in the group of 1.5×10⁷ CFU/g was significantly increased, there was not significant difference of the whole body moisture and crude ash content among all groups. The dry matter, protein, fat and ash deposition rate in the group of 1.5×10⁷ CFU/g are significantly higher than the control group. ③ Compared with control group, the intestinal lipase activity was significantly increased in the group of 1.5×10⁷ CFU/g and 1.8×10⁸ CFU/g and intestinal protease activity was significantly reduced in the group of 1.8×10⁸ CFU/g. Adding E. faecalis to feed had no significant effect on intestinal amylase activity. ④The mean corpuscular volume, hemoglobin concentration and the number of platelets in the group of 1.5×10⁷ CFU/g are significantly lower than the control group. The erythrocyte count was not significantly different among all groups. ⑤The serum cholesterol, glucose content and the content of malonaldehyde in the group of 1.8×10⁸ CFU/g were significantly reduced compared with control group. Compared with control group, the activity of alkaline phosphatase in the group of 1.5×10⁷ CFU/g was significantly increased, but the activities of glutamic pyruvic transaminase and glutamic pyruvic transaminase in this group were significantly reduced. In conclusion, the suitable concentration range of E. faecalis for GIFT tilapia feed is 7.5×10⁷–1.1×10⁸ CFU/g, but adding E. faecalishas an adverse effect upon the ability of carrying oxygen of blood.