Objective: The purpose of this article is to report mixed germ cell tumor, which revealed changes compatible with early transformation of dysgerminoma to endodermal sinus tumor(EST) through histogenetic considerations and immunohistochemical stains. Methods: Ovarian germ cell tumors were reviewed from files of Dept. Ob/Gyn. Seoul Paik Hospital fiom 1992.1 to 1996.12. Total of 5 cases include 4 pure dysgerminoma and 1 mixed germ cell tumars. All tissues were fixed in 10% neutral buffered formalin and embedded in paraffin and reviewed by two pathologists with immunohistochemical staining for cytokeratin, vimentin, AFP, PCNA, p53 & bc1-2. Results: Grossly, the areas of transformation were located at the middle of the mixed tumor. The outer layer of the tumor mass was filled with typical pure dysgerminoma. They were characterised as the presence of microcysts and small glandular structures in hematoxylin-eosin(H-E) stains with positive stain for vimentin, except the tissue of the EST. The cells in the intermediate layer were characterised as the mixed form of dysgerminomatous and EST structures in H-E stains. AFP in the dysgerminomatous cells in intermediate layer and EST were stained, but not in outer layer. Conclusion: Dysgerminoma may possess the ability to transform to EST. There might be intermediate stage between dysgerminoma and EST, and Immunohistochemical staining for AFP, cytokeratin, vimentin, PCNA also can be used for prognosis of germ cell tumor.