Kefir is a unique combination of lactic acid bacteria, yeasts and sometimes acetic acid bacteria that symbiosis in a natural polysaccharide and a protein matrix. During research on microorganisms and bioactive compounds of kefir, Many potential effects of this substance on facilitating the functioning of the gastrointestinal tract, treatment and prevention of gastrointestinal infections, protective effects against cancer, improvment on phagocytosis, and immune function, decreased blood sugar, blood lipid levels, hypertension and reduction on food allergy symptoms have been seen in kefir. In the present study, the effects of kefir powder on growth rate, immune system function, survival, carcass composition, intestinal microflora, hematology, biochemical parameters and liver enzymes of fingerling Nile tilapia (with average weight 17.3 ± 0.09 g) was investigated. Therefore, different levels of kefir powder were used, including control treatment (without kefir powder), 0.5, 1, 2 g/kg kefir powder in diets were evaluated in a completely randomized test. The results have shown a better effect of kefire powder on improving growth performance, specific growth rate, feed conversion ratio, weight gain and improvement of intestinal microflora of fingerling Nile tilapia at the level of 1g/kg kefir powder (p 0.05). According to the results, the best specific growth rate with a rate of 2.76± 0.01, the best feed conversion ratio with a rate was 0.96± 0.01. Also, survival was 100% in all treatments. On the other hand, the use of kefir powder at higher levels has increased the liver enzymes, therefore the level of 1 g/kg is desirable and recommended in terms of liver cell health and optimal liver function. The measurement of whole blood protein, albumin, globulin, dir="LTR" Keywords: Probiotics, Complement activity, Lysozyme, Nile tilapia, Kefir