Objective:To compare the effects of different dosage of dexamethasone (DEX) on rat lung morphogenesis. Methods:Total of 120 newborn rats were divided into three groups randomly:a small dosage DEX group,a large dosage DEX group and a control group. On the 10th,14th,21st,45th days after birth,the histologic structures of 10 rat lungs in each group were observed with light microscope. Results:Under the light microscope,during the first 2 weeks,rat lungs in the two treatment groups had fewer alveolar numbers,larger alveolar space,and thinner alveolar septum compared with those in the control group(P < 0.01). On the 21st and 45th days,the airspaces of rat lungs in the two treatment groups were still enlarged,while the alveolar spetums were thickening. An“emphysematous”condition of the lungs could be observed especially in the large dosage DEX group. Conclusion:Using DEX postnatal could promote lung morphogenesis obviously,while with the dose increased,the normal alveolization was inhibited,and the impact continued well into adulthood.