Non-ruminants Full-length research article Effect of different sources of selenium on performance and characteristics of immune system of broiler chickens ABSTRACT-An experiment was conducted based on a completely randomized design with 500 Ross 308 broiler chickens to investigate the effect of three different sources of selenium on performance and characteristics of immune system. Chickens were arranged in 10 treatments and five repetitions (10 chickens for each repetition). Ten treatments of experimental diet were considered in the study, including a basal diet (control); 0.5, 0.8, and 1.2 mg nano-selenium + basal diet (three treatments); 0.5, 0.8, 1.2 mg selenomethionine + basal diet; and 65, 80, and 100 mg vitamin E + selenium + basal diet. Findings suggested that adding selenomethionine (1.2 mg) to the basal diet resulted in a significant effect on feed conversion ratio and weight of broilers. In fact, weight and bursa of Fabricius were significantly under influence of the diets containing nano-selenium. Moreover, a significant difference was observed in the antibodies produced against Newcastle and influenza as well as the total immunoglobin and IgG produced against sheep red blood cell at two stages in the group treated with nanoselenium. Finally, results indicated that nano-selenium can better improve performance and immune system of broilers compared with diets containing selenomethionine and vitamin E.