Objective To explore the relationship between the expression level and localization of CD80 and carcinoembryonic antigen(CEA) in the human fetal thymus and its development. Methods Twelve aborted dead fetus samples(via donation from the dead fetus’s parents and permission of the hospital agency) were collected from Department of Genecology and Obstetrics in the affiliated hospital, including 4 cases aged less than 4 months in earlier stage and 8 cases aged 5 months in later stage. The two groups of earlier and later aged samples were randomly divided into two aliquots. One aliquot was prepared for frozen sections stained with HE, ANAE histochemistry, CD80 and carcinoembryonic antigen (CEA) immunohistochemistry; and detected with CEA ELISA. The protein extracted from another aliquot with Trizol reagent was detected with CD80 immunoblotting and CEA ELISA. Results The thymus in earlier aged group, especially the medulla was less developed. The reticulo-epithelial cells (RECs) with CD80IR in the later aged group could be classified into six patterns. The type I RE in flatten shape were located at the inner surface of the capscule and trabecula. The type II RECs (DCs) in small satellite shaped were localized in the medulla. Both type III and type IV RECs were located at the dzemarcation between the cortex and medulla. The type V RECs (DCs) in large satellite shaped with delicate processes to connect into reticular meshes were distributed in the medulla and the type VI RECs were found in the Hassall’s corpuscles. The ANAEdot pattern T cells (CD4SUP>+ /SUP>) and ANAE-diffuse granular pattern T cells (CD8SUP>+/SUP> ) in separate clusters could be found in the reticular pores. In the immunoblotting and ELISA the expression level of both CD80-IR and CEA-IR in the later aged group was higher than that in the earlier aged group.Conclusion The results suggest that the expression and cocalization of CD80 and CE