Stocking density (SD) is one of the key factors influencing the perceived level of stress in fish. The present study was designed to evaluate the effects of dietary graded levels of a new commercial probiotic Pro-Grow^® and SD on adult Nile tilapia Oreochromis niloticus, concerning rearing water quality, growth performance, feed utilization, and fish carcass composition for 15 weeks. Fish with an average initial body weight (55.75 ± 0.6 g) were distributed into six treatments (three replicates per treatment). Fish were stoked at 40 and 80 fish / m³ and fed Pro-Grow^® probiotic at three levels (0, 10 and 20 g / kg diet) for each SD. The obtained results revealed that the high SD had negative significant (P ≤ 0.05) effects on rearing water quality (dissolved oxyegen and total nitrogen ammonia, but still within the acceptable ranges for rearing O. niloticus), growth performance, feed utilization and fish carcass composition (ash and crude protein) parameters compared to the low SD. However, addition of dietary Pro-Grow^® significantly improved these adversely effects of high SD on fish, especially at high level 20 g / kg diet followed by 10 g / kg diet compared to the control group. Hence, the significantly interaction between high SD and high level of Pro-Grow^® was detected in most of the above parameters. So, it could be concluded that the high level (20 g / kg diet) of Pro-Grow^® probiotic is useful with high SD (80 fish / m³) for enhancing the production performance of O. niloticus.