The study was aimed to investigate the ultranstructural feature and diagnostic criteria of congenital dyserythropoietic anemia-type I (CDA-type I). Nucleated red cells in bone marrow from two patients with CDA-type I were analyzed by transmission electron microscopy (TEM). The results indicated that the erythropoietic/granulopoietic ratio was markedly increased with megaloblastic morphology in all stage of erythrocyte. Most proerythroblast showed of irregular nuclei, while the Swiss-cheese-appearance of the heterochromatin was usually found in basophilic and polychromatic erythroblast. About half of orthochromatic erythroblast illustrated karyolysis and karyorrhexis. Some orthochromatic erythroblast exhibited karyolysis and plasmolysis simultaneously. The inter-nuclear chromatin bridge between separated erythroblasts was seldom found by TEM. The nuclear membrane and rough endoplasmic reticulum were destructed at all stage of erythrocytes in different degree. In conclusion, the megaloblastic erythrosis was the main characteristic of CDA-type I, and then nuclear membrane disruption in polychromatic erythroblast and karyolysis or karyorrhexis in orthochromatic erythroblast. The universal breakdown of cytoplasm membranous system was fundamental pathogenesis of CDA-type I.