BACKGROUND: Outbreak and spread of bacterial infections resulted in increasing use of antibiotics which subsequently leaded to bacterial resistance to conventional antibiotics. Objectives: To evaluate the effects of organic acids (Selko-pH®), Probiotic (PrimaLac®) and Echinacea purpurea (EP) alcoholic extract on the immune system of the broiler chickens, a completely randomized design experiment was conducted over 120 one-day-old male broiler chicks (Ross 308) with 4 treatment groups that each treatment consisted of 3 replications with 10 birds in each pen. Methods: Groups were designed as follow: group one received organic acid constantly in drinking water (1:1000 v/v) until 14th day and then received 8 hours in a day consecutively (OA); group two received Probiotic (PrimaLac®) water soluble (by 14th day) and mix in the feed (until the end of the rearing period) (PM); group three received EP alcoholic extract in drinking water (1:1000 v/v) like organic acid group and group four were considered as control (received no feed additives). Results: All groups except the control increased the relative weight of the spleen and Bursa of Fabricius as lymphoid tissues; however, this increase was statistically significant only in the case of EP group (p≤0.001). Data of antibodies test against sheep red blood cells and heterophil to lymphocyte ratio also showed that all groups except the treatment of organic acid had significant differences in terms of specific and cellular immune responses (p<0.01). The data of total count of bacteria and also Lactobacilli bacteria in cecal’s content obviously showed that the number of Lactobacilli bacteria significantly increased in PM and OA treatments (p≤0.05). ConclusionS: Feed additives used in these experiments have the ability to improve the immune system.