Objective：To compare the effects of antenatal ambroxol and dexamethason on rat fetal lung morphogenesis. Methods：Nine pregnant rats were divided into three groups randomly：ambroxol group，dexamethasone group and control group with 3 rats in each group. On the 19th day on gestation， cesareans were taken out and the histologic structures of 6 fetal rat lungs of each pregnant rat were observed with light microscope，electronic microscope and image analysis． Results：Under the light microscope, compared with control group，fetal rat lung in the two treatment groups had more alveolar numbers, larger alveolar space, and thinner alveolar septum（P < 0.01）. Under the light microscope, compared with ambroxol group，fetal rat lung in the dexamethasone group had more alveolar numbers, larger alveolar space, and thinner alveolar septum（P < 0.01）. Under the electronic microscope, lamellar body and cellular organs such as bioblast in endochylema of the two treatment groups were more common compared with the control group. And lamellar body in the two treatment groups was more anachromasis and condensed compared with the control group． Conclusion：Both ambroxol and dexamethasone could promote the development of fetal rat lung， and dexamethasone had prior effects to ambroxol. However, considering of much side effects of dexamethasone, it is suggested that ambroxol could be used as a better drug to promote fetal lung maturity．