The ultrastructure of the megakaryocytes in the bone marrow from 14 cases of megaloblastic anemia was studied. The commonest change observed was that the development of the nucleus lagged behind that of the cytoplasm. There were some evidences supporting the theory of ineffective thrombocytopoiesis. The mechanism of the relationship between the changes of morphology and function based on the impaired DNA synthesis and methylation due to folate and vitamin B12 deficiency were also discussed.