Abstract: In this paper,the differentiation and development of ovary in the first sexcycle and the ultrastructure of different stages of oocytes in Clarias lazera have been studied.PGCs originally appeared in the endoderm of late gastrula,migrated from the endoderm to mesodermal segments and then successively to splanchnic mesoderm,to parietal mesoderm,to dorsal mesentery where PGCs eventually reached the genital ridges.The ovary differentiation happened 66 days age.The ovarian development in the first sexcycle has been divided into six stages.It needs about 136 days age for the fish to grow to be sexually mature.The ultrastructural feature of oocytes in different stages and the formation of yolk granules have also been discribed.