The objective of this study was to evaluate the effect of graded levels (0.0 , 2.5, 5.0 and 7.5g /kg diet ) of the dietary blue-green microalgae (Spirulina) Arthrospira platensis as feed supplement to assess growth performance , nutrient utilization, immune response , some hematological parameters, and proximate composition of the Nile tilapia (O. niloticus) . A completely randomized experimental design was developed with 4 treatments and two replicates . Fish (80 all-male Nile tilapia fingerlings) ,with an average initial weight of 6±0.20 g , were randomly distributed into 8 glass aquaria (112 liter) and each aquarium holding 10 fish and randomly assigned to one of two replicates of the diets and offered feed to satiation to fingerlings O. niloticus.  Fish were fed a balanced diet of 30% protein along the period of the experiment. The treatments were Diet 1 (Control) without Spirulina, Diet 2 (2.5g Spirulina /kg diet), Diet 3 (5g Spirulina /kg diet) and Diet 4 (7.5 g Spirulina /kg diet ). The results indicate that, the fish groups received Spirulina supplemental diets revealed significant improvement in growth parameters (body weight gain, feed conversion ratio and protein efficiency ratio). Since the diets contained microalga Spirulina at a level (7.5 gram/kg diet) showed the highest values of growth parameters and protein utilization. Carcass composition of the experimental fish was relatively affected by different dietary treatments. This study suggested that Spirulina at a level of 7.5 gram/kg can be used in the feed of Nile tilapia without negative effect on growth parameters, feed utilization and blood parameters.