Abstract： Pulmonary hypertension(PH) is a common complication of chronic obstructive pulmonary disease(COPD). In COPD, PH progresses slowly. It may firstly appear during exercise and during sleep,then at rest, and worsen during exercise, sleep or exacerbation. PH in COPD is usually mild to moderate. However,a small proportion of COPD patients may present with "out of proportion" PH. Cigarette smoke components or inflammation products may damage endothelial cells and produce endothelial dysfunction. Hypoxia may induce vasoconstriction. Pulmonary vascular remodeling leads to reduction of arterial lumen and distensibility of the pulmonary vessels, which increase pulmonary vascular resistance. Severe emphysema leads to reduction in the total number of pulmonary vessels. All these contribute to the development of PH in COPD.