Abstract: Histology methods and transmission electron microscope technique were used to investigate the pulmonary morphological structure and adaptive changes in one-day-old yak, five-month-old yak and adult yak. The results showed that the percentage of media muscle thickness of pulmonary artery (MT%) were thicken, the mean value of one-day-old group, five-month-old group and adult group were 10.71%, 12.53% and 11.18%, respectively. There was a complete smooth muscle layer in bronchiole wall in one-day-old yak. In gas-conducting airways, the secretory granules of goblet cells were dense; there was sparse area in granules central. The secretory granules of Clara cells were dense secretory granules or a mixture of dense secretory granules and pale secretory droplets. In pulmonary artery, endothelial cells were cubic and proliferated, some of them intruded lumen. Smooth muscle cells are cubic, organelles increased. The thickness of blood-air barrier in yak was very thin. The thickness of blood-air barrier in different age group were 0.445(one-day old),0.506(five-month old) and 0.423(adult) μm, respectively. These results indicated that hypoxia affected the structure of lung in yak; it was significant in five-month-old group, but as the increasing of age, the affection was decreasing. The changes of structure affected with age indicate that the lung of yak has adapted to the hypoxia.