Abstract: The formation and development of archegonium in Ceratopteris thalictroides(L.) Brongn has been studied mainly using transmission electron microscopy(TEM).The results revealed that archegonium in C.thalictroides was generated from an initial cell of gametophyte.The initial cell formed three layers by two divisions.Cells in the top and lower layers give rise to the jacket cells of archegonium.The middle cell is named primary cell,which is the first cell in oogenesis.This cell ultimately developed into a binucleate neck canal cell,a ventral canal cell and an egg cell by two unequal divisions.This research specified the sequences and characteristics of the archegonium interior cell in C.thalictroides.