The present study was implemented to evaluate the effect of probiotic on improving water quality, growth performance and body composition of Nile tilapia. However, probiotic (EM.1®) was added to rearing water at levels of (0.0 ppm, 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm and 500 ppm). Water analysis indicated that probiotic application at level of 200 ppm in rearing water significantly improved Dissolved Oxygen (9.02±0.48 mg/l), while it decreased ionized ammonia (0.77±0.03 mg/l) and Un-ionized ammonia (0.04±0.01 mg/l). The maximum fish growth and the best food conversion ratio (1.49±0.07) were obtained at level of 200 ppm. Chemical composition of whole-body fish was significantly affected by probiotic adding to rearing water. The best protein content (13.85±0.21) was obtained at level of 200 ppm. Thus, the present study recommends using probiotic (EM.1®) in rearing water with level of 200 ppm to improve water quality and to enhance fish productivity.