The present study was conducted to investigate the effects of N-acetylcysteine (NAC) on antioxidant capacities and the expression of inflammatory cytokines in weaned piglets. In total, 15 litters, containing 150 14-day-old piglets, were divided, by litter, into suckling (control), weaning and NAC groups. Thus, there were five litters per group. Compared with the suckling group, the weaning group had increased (p < .05) malondialdehyde (MDA), nitric oxide (NO), hydrogen peroxide (H2O2) and interleukin (IL)-6 contents in their serum and pro-inflammatory cytokine IL-6 and IL-1β expression levels in the jejunum, while they had decreased superoxide dismutase (SOD) (p = .055) and glutathione peroxidase (GSH-Px) (p = .057) activities, as well as IL-4 (p < .05) concentrations in their serum and I-κBα expression (p < .05) in their jejunum. Dietary supplementation with NAC had improved (p < .05) antioxidant status in serum and increased (p < .05) levels of IL-4 but decreased (p < .05) mRNA expression levels and concentrations of IL-6 and IL-1β. Meanwhile, the I-κBα expression level in the jejunum of the NAC-treated group increased (p < .05) compared with the weaning group. In conclusion, weaning resulted in oxidative stress, and dietary supplementation with NAC could increase the antioxidant capacities, while decreasing the jejunal expression of inflammatory cytokines in weaned piglets.