Objectivs To study the effects of dexamethasone in different time on lung pathological morphology changes and growth development in neonatal rats which injury of hyperoxia, in order to providing the theoretical reference of dexamethasone to prevent and cure bronchopulmonary dysplasia (BPD). Methods The Wistar neonatal rats were designed in eight groups by random including:air group;hyperxia group;hyperxia group with early dexamethasone group and later dexamethasone group.Body weight, general situation and the death number of rats, the degree of pulmonary edema, alveolarization block and pulmonary fibrosis from lung coefficient calculation, alveolar interval thickness, radical alveoli count (RAC)and pulmonary fibrosis stocker score were recorded. Results The hyperxia group rats showed serious lung injury after exposed 95% oxygen.The general situation of early dexamethasone group rats was the worst and the degree of pulmonary edema, alveolarization block and pulmonary fibrosis were the most serious.The general situation of the later dexamethasone group was better and the growth increased faster than hyperxia group.But there had not significant differences in lung coefficient calculation, RAC, alveolar interval thickness and pulmonary fibrosis stocker score between the later dexamethasone group and the hyperxia group. Conclusions Early use dexamethasone has no action to prevent BPD, instead it may aggravated the hyperxia lung damage.Dexamethasone couldn't suggest as prophylaxis for BPD, dexamethasone should be used in the late therapy stages, and as small doses and short treatment as possible.