Objective To investigate the expression and effect of elastin in full-term rats with hyperoxia induced by chronic lung diseases. Methods One hundred and forty-four full-term rats are randomly exposed to hyperoxia (hyperoxia group)and room air(room air group).Chronic lung disease(CLD)is induced by hyperoxia exposure. Gomori's stain for elastic fibers and in situ hybridization methods were used to detect the expressions of secondary crest and tropoelastin mRNA on the 1st,3 rd, 7th, 10th,14th and 21st days after exposure. Results The expressions of secondary crest decreased significantly in hyperoxia group, compared with room air group on the 3rd to 14th days(P＜0.05).The expressions of tropoelastin mRNA decreased significantly in hypemxia group, compared with room air group on the 3rd to 10th days (P＜0.05),otherwise increased significantly from the 14th to 21 st days(P＜0.05). Hyperoxia exposure can delay the peak of tropoelastin mRNA. Conclusion Elastin is involved in the inhibition of alveolarization and lung fibrosis in the development of CLD.