In this paper,the structure of the root tubers of Ophiopogon japonicus and Liriope spicata at different developmental stages was observed under light microscope by means of the method of paraffin section.The results indicated that root tubers enlargement of O.japonicus and L.spicata was all initiated from the apical part of adventitious root and that both the cell layer number and cell size in cortex changed dramatically during development in both species,which finally enlarged the root tubers.But we found that the cell layer number increasing in L.spicata contributed more than cell size change to the root tuber enlargement,while in O.japoriicus both cell layer number and cell size contribute equally to the root tuber development.