Were investigated microscopical structure features of domestic ducks' (Anas platyrhynchos domesticus) small intestine of the black-white-breasted breed of Ukrainian selection in 9 age groups: 1-, 3-, 7-, 14-, 21-day, 1, 2, 6-month, and 1-year-old. The microstructural morphometric parameters were determined on histological preparations from cross-sections of the intestine's middle part. the surface area was calculated by determining their height and width. The density of villi and crypts was identified based on 1 mm of mucosal length. A common pattern in the dynamics of most morphometric indicators (except for the density of villi and crypt) of the small intestine wall was their increase. Such changes in the size of the intestinal microstructures did not occur simultaneously, they were asynchronous. In some cases, they decreased if compared to a younger age. The most rapid increase in the microstructures of the duodenum, jejunum, and ileum occurred in the first month of the postnatal period of ontogenesis, less intensively in the second month. During the first month, these indicators were changed most rapidly during the first week. They corresponded to the indicators' value of an adult bird at different ages. Thus, the thickness of the muscle layer and density of the duodenal crypts corresponded to the value of adult birds at 21-days-old, the intestine's diameter and wall thickness at 1 month, the mucous layer thickness, the height and density of the villi, and the depth of the crypts at 2-month-old. The wall thickness, mucosal and muscular layers, the height of the jejunal villi corresponded to those in the adult birds of 1-month-old, the diameter of the intestine, the crypts' depth, the density of the villi at 2-month-old, the density of the crypts at 6-month-old. The density of the ileum crypts corresponded to the adult bird at 7-days-old, the intestinal diameter was like at 21-days-old, the depth of the crypts was like at 2-month-old, the density of villi was like at the age of 1-month-old, the intestine wall thickness and muscle layer were like at 6-month-old, the mucous membrane thickness, the height of the villi were like at the age of 1 year.