A total of, 200, day old Hubbard broiler chicks, were randomly divided into four treatment groups (n = 50) with five replicates (n = 10) in each group and allotted four treatments. Birds in the control group (β-GOS-0) were given a corn based diet, whereas, birds in other three experimental groups were given same diets supplemented with different concentrations of β-GOS i.e., 0.1% (β-GOS-1), 0.2% (β-GOS-2) and 0.5% (β-GOS-5) for a period of 35 days. The birds were vaccinated against the Newcastle disease virus. Moreover, birds were also sensitized against 2, 4-dinitrochlorobenzene for determination of cell mediated immunity. At the end of experiment, ten birds from each group were slaughtered to collect blood for serum biochemical metabolites and humoral immunity determination and caecal digesta for selected caecal microbiota enumeration. Data were analyzed using one-way ANOVA. In case of significant F-value (P < 0.05), Tukey's post hoc test was applied. Results demonstrated that cell mediated immunity was higher (P < 0.05) in the β-GOS-1 group. Concentrations of caecal Lactobacilli were higher in the β-GOS-2 and β-GOS-5 groups. The serum biochemical metabolites, humoral immunity and counts of Clostridia and coliforms were not affected by dietary supplementation of β-GOS. In conclusion, dietary supplementation of β-GOS conferred better effects in terms of increased caecal Lactobacillus counts and immuno-modulation in broilers.