Abstract: 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

Key words: 1-day old, Pulmonary alveoli, Blood-air barrier, Plateau hypoxia adaptability, Transmission electron microscopy, Morphometry, Yak, Cattle