Selenium (Se) is an essential nutrient that plays important role in numerous biological processes and fish feed is the major route of Se supply to fish. A 70-days feeding trial was conducted to evaluate the effects of selenium nanoparticles (Se-NPs) on growth, hematology, and body composition of Oreochromis niloticus fingerlings. A total of 270 fingerlings were assigned into 6 treatment groups. Se-NPs (%) were added to formulate six test diets viz. D-0 (control), D-0.1, D-0.2, D-0.3, D-0.4 and D-0.5, respectively and fed to fingerlings under completely randomized design (CRD). Growth parameters such as weight gain % and FCR started to improve (p≤0.05) when fingerlings were fed on test diet D-0.1 and reached to maximum (229.74%, 1.29) when fish fed on diet D-0.3 as compared to fish fed on control diet (D-0). The values of hematological parameters such as red blood cells (2.90×10⁶mm^-3) and white blood cells (7.89×10³mm^-3) were found to be the maximum (p≤0.05) at D-0.3, while hemoglobin (7.95 and 8.47) was found to be improved significantly at D-0.2 and D-0.3, respectively. Similarly, parameters of body composition i.e. crude protein (20.86%) was improved significantly at D-0.3 while crude fat showed non-significant improvement at D-0.2 and D-0.3 (3.10% and 2.17%) respectively as compared to the control diet. Above D-0.3 level, a significant increase was observed. Fat and moisture contents were found to be maximum at control and D-0.5. So, the results suggested that Se-NPs have the potential to improve the fish body composition and are recommended to be used in fish feed at an optimized level.