Two experiments were conducted to investigate the effects of supplemental glutamine on growth performance, plasma parameters and LPS-induced immune response of weaned barrows after castration. In experiment 1, forty-eight weaned male piglets were used and fed maize and soybean meal diets supplemented with 0 (Control) or 2% L-Gln (Gln+) for 25 days. The results indicated that the Gln+ group tended to increase average daily gain compared to control in stages of days 7 to 14 and 0 to 25. The Gln+ had significantly better feed efficiency than the control group did during days 14 to 25 and 0 to 25. The plasma blood urea nitrogen and alkaline phosphatase contents of Gln+ group were higher than those of the control group on day 14 post-weaning. In experiment 2, sixteen weaned male piglets were injected with E. coli K88+ lipopolysaccharide (LPS) on day 14 post-weaning. The results showed that the Gln+ group had lower concentrations of plasma adrenocorticotrophic hormone and cortisol than the control group on day 14 pre-LPS challenge. In addition, Gln+ group had higher plasma IgG concentration than the control group for pre- or post-LPS challenged on day 14 post-weaning. In summary, dietary supplementation of Gln was able to alleviate the stressful condition and inflammation associated with castration in weaned barrows, and to improve their immunity and growth performance in the early starter stage.