The objective of these studies was to evaluate the effect of supplying mannan oligosaccharides (MOS) to pig diets on both weanling pig and sow performances. Nursery pigs challenged with LPS to stimulate an immune challenge had increased body temperature and respiratory rate as well as elevated serum cortisol and TNF-α concentration, MOS-supplemented pigs had a lower rectal temperature and respiratory rate which implied that MOS improves some aspect of the immune function of piglets. MOS supplementation in sow diets during late gestation and lactation had no effect on litter size, but the piglets from MOS-fed sows were heavier at birth (P = 0.04), at weaning (P = 0.03), and during the entire nursery period (P < 0.01). Moreover, milk fat and protein levels as well as the Ig concentrations in milk from MOS-fed sows were numerically higher (2 – 12%; P > 0.10) than control sows. Overall, MOS supplementation in the nursery diet may have limited effects on the growth performance, but may have some beneficial influence on pigs under immune challenge. Furthermore, including MOS to the sow diet during late gestation and lactation can potentially improve piglet body weight as well as growth during the suckling and nursery periods.