Following the maturation of follicular cells, the zona pellucida, the oocyte nucleus and cytoplasm, oocytes are ready for fertilization and further embryonic development. The mechanism of nuclear maturation has been investigated previously, however, cytoplasmic maturation has not been described clearly. From our ultrastructural observation, we defined cytoplasmic maturation as development of smooth endoplasmic reticulum, aggregation and development of mitochondria, and extension and ramification of microvilli on the surface of human oocytes. Furthermore, metaphase I oocytes, which show no specific features under light microscopy, have been found to undergo important changes in the whole maturation process. In metaphase I oocytes, organella such as mitochondria and smooth endoplasmic reticulum develop well and the communication between follicular cells is initiated. Thus, ultrastructural study may contribute to the elucidation of the complicated maturation process of human oocytes during in vitro maturation (IVM).