Abstract: This experiment aimed to study the relationship between hypoxia and expression of c-fos,c-myc in broiler’s pulmonary arterial smooth muscle cell (PASMC). The expression of c-fos,c-myc were observed by in situ hybridization, immunohistochemistry and image analysis methods. The results showed that hypoxia can significantly increase c-fos and c-myc mRNA expression(c-fos mRNA: 144.6±20.2 in normal oxygen group, 198.1±32.8 in hypoxia group，P<0.01; c-myc mRNA: 125.4±18.8 in normal oxygen group, 167.1±22.4 in hypoxia group，P<0.01).Hypoxia can significantly increase c-fos and c-myc protein expression(c-fos protein: 150.9±33.2 in normal oxygen group, 225.9±37.0 in hypoxia group; c-myc protein: 162.1±28.5 in normal oxygen group, 228.8±33.4 in hypoxia group, P<0.01).The results indicated that hypoxia play an important role in the PASMC proliferation. As pulmonary arterial remoulding were the main characters of ascites syndrome（AS）, so this experiment indicated that hypoxia is the important cause of AS.