The present study was aimed to investigate the effects of probiotic and hydrolyzed tannins on performance, the relative organ weights, circulating differential leucocyte count, humoral immune responses, intestinal pH and microbial population alterations of broiler chicks. For this purpose, 1404 one-day-old male Ross broiler chicks were randomly allocated into nine experimental treatments with six replicates of 26 birds each. The experimental treatments consisted of three levels of probiotic (0.0, 0.1 and 0.2 g/kg) and three levels of hydrolyzed tannins (0.0, 0.75 and 1.5 g/kg) in a 3×3 factorial arrangement administered during a 42-d feeding trial. Our results showed that the effect of using different levels of tannins and probiotics and their interactions on feed intake in different experimental periods were not significant (P 0.01). The interaction effects of probiotic and tannin supplements had the significant effects on average daily weight gain and feed conversion ratio during the growth, finisher and whole trial periods (P 0.01). So that, the highest average daily weight gain and the lowest feed conversion ratio were obtained in broilers fed on 0.75 g/kg tannin and 0.1 g/kg probiotic. Also, the main effect of using different levels of tannin in all experimental periods on average daily weight gain was significant (P 0.01). So that the highest average daily weight gain was related to the birds supplemented with the treatment of 0.75 g/kg tannin level. In the starter period and the whole experimental period, the use of 1.5 g/kg tannin level had an adverse effect on feed conversion ratio (P 0.01). Although the relative weights of heart, liver, gizzard, fat, pancreas and intestine were not affected by the interactions of probiotics and tannins, the weights of spleen and bursa of fabricius were significantly affected (P 0.01). The results showed that feeding 1.5 g/kg tannin increased the ratio of heterophils to lymphocytes when compared to the control group (P 0.01). Also, dietary inclusion of 0.2 g/kg probiotics significantly reduced the ratio of heterophils to lymphocytes as compared to the control group, and it increased the percentage of monocytes (P 0.01). Interactions between different levels of tannins and probiotics on humoral immune responses showed that IgM titer during the primary immune response as well as total antibody titer and IgG titer in secondary immune response against SRBC were significant (P 0.01). The use of 0.75 g/kg tannin improved the total antibody titer and IgM in the primary immune response, and also the total antibody titer and IgG titer in the secondary response against SRBC (P 0.01). However, feeding the double level of tannins (1.5 g/kg tannin) significantly decreased these indices. Also, the study of the main effects of different levels of probiotics showed that dietary inclusion of probiotic had no significant effect on the titer of antibody against sheep erythrocytes. The main effects of different levels of tannins and probiotics on ileal micrflora and small intestinal pH value were significant (P 0.01), so that different levels of tannins (0.75 and 1.5 g/kg) and probiotics (0.1 and 0.2 g/kg) significantly depressed the Escherichia coli population when compared to the control group. Also, increasing levels of probiotics (0.1 and 0.2 g/kg) significantly (P 0.01) increased the population of ileal Lactobacillus as compared to the control group. The pH level of ileal content was significantly (P 0.01) decreased by feeding the incremental levels of tannins and probiotics when compared to the control group. The interactions between the mentioned factors on the small intestinal histomorphology were significant (P 0.01). The main effects of tannin levels showed that dietary supplementation of 1.5 g/kg tannin markedly decreased the villi height, the ratio of villi height to crypt depth and also increased crypt depth (P 0.01). The main effects of probiotic levels also showed that increasing probiotic levels led to an increase in the goblet cells counts (P 0.01). The results show that although feeding 0.1 g/kg probiotic decreased the numbers of lamina propria lymphoid follicles (LLF) (P 0.01), but dietary administration of 0.2 g/kg probiotic increased the LLF in when compared to the control group (P 0.01). In general, the findings indicated that although dietary inclusion of 0.2 g/kg probiotic improved performance and health indices, feeding 0.1 g/kg probiotic and 0.75 g/kg hydrolyzed tannin manifasted the best synergetic effects on growth performance, immune responses, ileal microflora, and jejunal morphology indices in broiler chicks. Keywords : Broiler Chicks, Probiotics, Hydrolysed Tannins, Immune Responses, Ileal Microflora, Small Intestinal Histomorphology