This study was undertaken to evaluate the use of spirulina (Arthrospir platensis) as immunity promoter for Nile tilapia, Oreochromis niloticus (L.).A total of 270 fish (50±5 g) were randomly distributed into six groups each at a rate of 15 fish per aquarium and fed on a diet containing 0.0, 5.0 or 10.0 g spirulina/kg diet for 6 weeks. Each subdivided into three equal replicates. After the feeding trial, fish of each treatment were challenged by pathogenic Pseudomonas. fluorescens which was given by intraperitoneal (IP) injection. The blood samples were taken after 4and 6weeks for immunological examinations. The results showed that the highest white blood cells (WBCs) ,neutrophils , monocytes and basophils were obtained at 5.0 - 10.0 g spirulina/kg diet before and after the infection. There were non significant changes in lymphocytes after 4 weeks, while after infection with Pseudomonas. fluorescens lymphocytes increased in groups supplemented with spirulina. Moreover, spirulina enhanced serum lysozyme activity, bactericidal activity, and antioxidant enzymes (GPx and SOD) of treated groups before and after the infection. Total fish mortality 10days were decreased after IP injection with Pseudomonas. fluorescens with the increase of spirulina level in fish diets. The lowest fish mortality was obtained when fish fed 10.0 g spirulina/kg. These results indicate that spirulina supplementation is promising for disease prevention in tilapia culture, and the optimum level of spirulina in fish diet is 10.0 g per kg diet.