The ultrastructure of the sporoderm and the dynamic of polysaccharides and lipid droplets during the sporangium development in the fern Osmunda japonica Thunb. were studied using light microscopy, transmission electron microscopy and cytochemical techniques. The results showed that: (1) the sporangium of O. japonica consists of a layer of sporangium wall cells, 2 layers of tapetal cells and archesporial tissue; (2) the sporoderm consists of exospore and perispore. The exospore, which can be divided into the outer exospore and inner exospore, is thick obviously. But, the perispore is thin and discontinuous. The ornamentations is short baculate and formed by the exospore. The inner exospore is composed of polysaccharides. Both outer exospore and perispore contain lipid materials. (3) Some lipid droplets are seen in the archesporial cells. As the sporangia develop, the amyloplasts in the spore wall cell degenerate. The size of the amyloplasts become small and the number of the amyloplasts decrease gradually. Then these amyloplasts are transported to the plasmodium of the tapetal layer. The amyloplasts are probably transformed into the precursors of sporopollenin and sporopollenin. The sporopollenin are converted into spherical bodies, which pass through the inner membrane of the plasmodium and enter to the sporangium cavity. Finally, these sporopollenin bodies are deposited on the surface of the spores. (4) during spore development, the polysaccharide materials are converted into lipid materials, which are stored in the spores in the form of lipid droplets. The metabolic characteristics of polysaccharides and lipid droplets are observed during the sporangium development, which provided fundamental data and information for studying sporogenesis.