ABSTRACT A feeding trial was conducted to investigate the effects of dietary supplementations of synbiotic and probiotic on broiler performance, carcass yield, organs weights, and histomorphological measurements of small intestine. Six hundred 1-d-old broiler chicks were randomly assigned to 1 of 3 dietary treatments for 5 wk. The dietary treatments were 1) control, 2) basal diets supplemented with synbiotic (1 kg of Biomin IMBO/ ton of the starter diets and 0.5 kg/ton of the grower diets), 3) basal diets supplemented with probiotic (1 kg of a homofermentative and a heterofermentative Lactobacillus sp./ton of feed). The BW, average daily weight gain, carcass yield percentage, and feed conversion rate were significantly (P < 0.05) increased by the dietary inclusion of the synbiotic compared with the control and probiotic-fed broilers. Moreover, a slight improvement in performance traits was observed in broilers fed the probiotic compared with control birds. The absolute and relative weight of spleen and thymus tended to be greater (P < 0.1) for the probiotic-supplemented group compared with the synbiotic-supplemented group. The relative liver weight was greater (P < 0.05) for probiotic-fed birds compared with synbiotic-fed birds.