The present experiment investigated the effects of galacto-oligosaccharide (GOS) on growth performance, faecal microbiota, immune response and antioxidant capability in weaned piglets. Ninety 28-day-old weaned piglets were randomly allocated into 5 treatments with 6 replicate pens per treatment and 3 piglets per pen (n = 18). These five diets were formulated by adding 0, 500, 1000, 1500 and 2000 mg/kg GOS to the basal diet, respectively. The experiment was lasted for 28 days. Body weight and feed intake were determined. Faeces samples were collected to detect the amount of microbiota. Blood samples were used to determine antioxidative and immune parameters. The results indicated that GOS supplementation improved the growth performance, increased the number of Lactobacillus and Bifidobacteria and decreased the number of Escherichia coli in a linear or quadratic dose-dependent manner. Dietary GOS decreased serum concentration of pro-inflammatory cytokines in a quadratic dose-dependent manner, but increased anti-inflammatory cytokines in a linear or quadratic dose-dependent manner. Moreover, GOS supplementation promoted the activities of antioxidant enzyme in a linear or quadratic dose-dependent manner during the experiment. The levels of MDA quadratically decreased on d 28. These results suggested that GOS exhibited positive effects on growth performance, immune function and antioxidant capability in weaned piglets.