This study was carried out to evaluate the effects of different levels of crude protein (CP) and threonine (Thr) on performance, carcass traits, humoral immunity and intestinal morphology of broiler chicks. A total of 432 male broiler chicks (Ross 308) were randomly allocated in a factorial arrangement (3×3) based on a completely randomized design consisting 9 treatments with 4 replicates and 12 birds per replicate. treatments included 3 levels of CP (100, 95 and 90% of broiler Ross 308 recommendations) and 3 levels of dietary Thr (100, 110 and 120% of broiler Ross 308 specifications). Chicks recived starter (1-12 days), grower (13-24 days) and finisher (25-42 days) diets. Weight gain, feed intake and feed conversion ratio were determined over 3 age periods. On day 16, all birds were challenged by injection route with a commercial Newcastle vaccine. Then, on day 28, two birds from each replicate were chosen at random and blood samples were collected from the brachial vein and antibody titer against Newcastle disease viruse were performed by Hemagglutination halter test. Also, at 23 and 33 days of age, two birds from each pen were immunized intramuscularly into each thigh with 0.5 mL of 7% SRBC (Sheep Red Blood Cell) suspension to determine primary and secondary humoral antibody responses. Heparinized blood was collected from the birds at 6 day after first and 12 day after second immunizations with SRBC to evulate antibody titers against SRBC. On day 45, two chicks were randomly killed from each replicate to determine the effects of dietary treatments on intestinal cell morphology. At the end of experiment, two birds from each replicate were slaughtered and carcass, drumsticks, breast, pancreas, abdominal fat as well as lymphoid organs including spleen and bursa of fabricius were weighted and expressed as a percentage of live body weight. Experimental data were analyzed using the GLM procedure of SAS software. This study showed that the reduction of dietary CP decreased relative carcase yield and relative weights of abdominal fat and pancreas. However, reducing CP up to 10% had no effect on the relative weight of drumsticks and breast, but after addition of second level of Thr, these economic traits increased significantly (P?0.05). Reducing dietary CP, decreased primary (P=0.16) and secondary (P=0.13) immune responses against SRBC; however, after addition of 110% of recommended Thr, these responses raised (P 0.05). Although, after reduction of dietary CP level, the jejunal villus height, crypt depth and goblet cell numbers decreased significantly, but administration of diets with 110% of Thr requirement could compensate these alterations by enhancing the intestinal absorptive surface area and goblet cell numbers. After starting period, reduction of dietary CP up to 90% of recommended CP level had no suppressive effect on broiler growth performance, however, the best result obtained when diets administrated with 110% of recommended Thr. The present results indicated that reduction of dietary CP up to 10%, after starting period, had no inadvisable effect on growth performance, however, the best results obtained when diets supplemented with 110% of recommended Thr. Key words Broiler chicks, Threonine, Performance, Humoral immunity, Mucosal morphology