Abstract: This study was conducted to evaluate both physiological and immunological efficiency and toxicological effects of different selenium (Se) sources and levels in corn-soybean meal broiler diets. For that, 400 day-old unsexed Arbor Acres broiler chicks were allocated to 10 experimental treatments in a 5 sources x 2 levels factorial design. Five Se sources were tested; (1) sodium selenite (NaSe) as inorganic form; (2) selenomethionine (Se-Yeast) as organic form; (3) Zinc-L-selenomethionine (Zn-Se-Meth) as more recent organic form; (4) powder form of Nano Se form (P-Nano Se) and (5) Liquid form of Nano Se (L-Nano Se). Also two inclusion of Se levels in diets; 0.15 and 0.30 ppm, were examined. The inorganic and organic forms of Se were obtained from commercial suppliers while both powder and liquid forms of Nano Se were prepared immediately before starting feeding phases of the experiment. The prepared 80 nm Se nano-particles were synthesized by chemical reduction method and characterized by Transmission Electron Microscope, X-ray diffraction and spectrophotometry. Three phases (1-10, 11-24 and 25-40 d) of feeding were applied and all birds were kept under similar management conditions. Parameters of blood picture, immunity status, antioxidant status, some plasma constituents and T3 hormone were investigated. Also histological examination of liver samples was carried out at 40 days of age. The obtained results showed significant improvement of some hematological parameters, cellular immunity and antioxidant status either due to using organic or nano forms of Se, or by increasing the inclusion Se level from 0.15 to 0.30 ppm in broiler diets. While humeral immunity against Newcastle Disease Virus and Avian flow Virus (H5N1), plasma proteins, activity of liver enzymes and malnodialdhyde (MDA) content in plasma did not affected due to Se sources or levels. Concentration of T3 hormone significantly increased by increasing Se level from 0.15 to 0.30 ppm in the diet. The histological examination of liver showed some severe pathological changes due to increasing Se level from 0.15 to 0.30 ppm for most sources while using 0.15 ppm of Se from inorganic or organic forms of Se showed normal histological structure of liver tissues. The overall experimental results showed although using Zn-Se-Meth as organic form of Se or L-Nano Se as nano form of Se or increasing the supplemental Se to 0.30 ppm in broiler diets or its equivalent in drinking water is more effective to get better, physiological, immunological and antioxidant status of broiler chicks. Inclusion Se-Yeast as organic form of Se in broiler diets at level 0.15 ppm was more save to liver tissues and kidney function. Further studies about the safety of using nano form of selenium as feed additives are needed.