Objective: To study morphological evolution of fetal lung in SD rats and establish continuous systematic morphological datum. Methods: Fetal lungs at embryonic day16(E16),E19,E21,were taken out respectively,and were observed by HE staining under light microscope and electric microscope. Results: At E16,epithelial tract began to appear,which was constructed by tall epithelium,and developed many airway branches. At E19,alveolar space extended gradually,then lamellar body and type-II pneumonocytes began to emerge. At E21,alveolar separation increased gradually,and mesenchyma turned into thiner than before. At the same time the number of lamellar body increased gradually. Conclusion: The fetal lung development is a gradual process. In fact,this process will continue to postnatal stage. The appearance of lamellar body is the sign of differentiation of type-II pneumonocytes,and it increases along with lung development.