The objective of this study was to determine the Isoleucine requirement in early (EG) and late (LG) gestation using the indicator AA oxidation method. Each sow received 6 diets based on corn, corn starch and sugar in both EG and LG at constant amount of 2.5 kg/d. Diets in EG contained Ile at 20, 40, 60, 80, 100 and 120 % of the Ile requirement (6.2 g/d, NRC 1998) for sows of similar body weight (BW), expected maternal gain and litter size, and 60, 80, 100, 140, 160 and 180% in LG. Dietary valine and leucine contents were 3.5 g/kg and 4.3 g/kg in EG and 5.6 g/kg and 6.9 g/kg in LG, respectively. Ile requirements of adult sows increased more from EG to LG than tryptophan requirements in young, growing sows, and were driven mainly by maintenance in EG and by fetal growth in LG. The AA supply is more critical in LG than energy intake for adult sows. Therefore, phase feeding during gestation is necessary to meet the sows’ requirements.