Abstract: Fine structure of the yak lung was examined by microscopy and trans electro-microscopy, and then, the relationship between the morphological features and the adaptability to lower-oxygen was analyzed. The lobuli pulmonum occurred obviously. Most of the alveoli wall composed by type I alveolar cells which had gaps on it clearly. The arithmetic and conciliate averages of the air-blood barrier thickness were 0.53±0.10µm and 0.44±0.07µm respectively, which were notabilitly smaller than that of other smaller livestock. Those can facilitate the dispersion of oxygen through the barrier. The rate of muscle-layer thickness to outer diameter of the blood vessel was 5.00±0.93%, being similar as the camel house but smaller than the cow and bull, which can reduce pulmonary artery high pressure. There were lots of elastic fibres in pulmonary pleura, interlobular connective tissue, alveolar septa, the walls of different bronchus and blood vessels， which formed a complete elastic system in favor of the lung dilating and shrinking. Goblet cells that distributed not only in different bronchi but also in bronchioles, were good for dry air passing through the tract. These structural features were obtained by yaks as a result of their living in lower-oxygen on Qingzang Plateau from generation to generation.

Key words: Yak (Bos grunniens), lung, structure, adaptability