High productivity associated with the meat quality and low costs are the main objectives of meat poultry production. Removal of antibiotics from poultry diets in many parts of the world has amplified the interest in using feed additives in commercial poultry production to improve intestinal health and nutrient utilisation. We have conducted an experiment to assess whether the dietary inclusion of a microbial feed additive (MFA) has an effect on the small intestinal micro-architecture and oxidative stress biomarker (thiobarbituric acid reactive substance, TBARS) in the liver and brain of broiler chickens. Two dietary treatments (n = 10/treatment), consisting of a basal diet (control group) or the basal diet supplemented with 2.5 kg of MFA/ton of feed (MFA group), were fed to broiler chicks from 1 to 35 days of age. On day 35, birds were slaughtered to collect tissue samples from the duodenum and jejunum for histo-morphometry and from the liver and brains for measuring TBARS. The results showed that addition of MFA to the broiler diet only increased (P = 0.019) the crypt depth of the villi in the duodenum, without affecting the villus height, villus width and thickness of the muscularis of the duodenum and the jejunum. Furthermore, supplementation of MFA in the diet decreased the level of TBARS in the brain (P = 0.013) and in the liver (P = 0.017), suggesting a decrease in the level of lipid peroxidation and oxidative stress. We conclude that the addition of microbial supplement to broiler diets had limited effects on the intestinal integrity but has the potential to alleviate the level of oxidative stress in broilers.