Objective To find out the characteristics of pulmonary alveoli histological structure and the adaptive mechanisms of 1-day old yak and its relations to the plateau hypoxia environment. Methods To analyze pulmonary alveoli of five 1-day old yaks and cattle respectively by HE staining, transmission electron microscopy(TEM) and morphometry. Results Pulmonary alveoli of 1-day old yak dilated sufficiently more than that of the cattle. There were massive sac-like structures which resemble alveolar sac in the lung of 1-day old yak. There was large bud-like septum in each saclike structure. The saclike structure was rare in the 1-day old cattle’s lung. Type Ⅱ alveolar cells were abundant in the 1-day old yak alveolar wall, and lamellar bodies were obvious in the yak alveolar cavity. Mean alveolar number(MAN) of 1-day old yak is less than that of the 1-day old cattle’s(EM>P/EM><0.05);but mean single alveolar area (MSAA), alveolar area in the unit area (SA) and mean thickness of alveolar septum(MSAT)in the yak alveoli were higher than cattle’s corresponding values(EM>P/EM><0.05). Arithmetic mean thickness of 1-day old yak blood-air barrier was thicker significantly than that of the 1-day old cattle’s(EM>P/EM><0.05)，but harmonic mean thickness of blood-air barrier was not significant (EM>P/EM>>0.05). Conclusion The development and growth of alveoli of 1-day old yak is more mature than that of the cattle’s. Yak alveoli increase rapidly by way of bud growth. Comparatively mature development and growth of lung，rapid increase in