The present investigation examines the oogenesis of the gleichenioid fern, Diplopterygium glaucum (Thunb. ex Houtt.) Nakai using transmission electron microscopy. The results demonstrate that the oogenesis of D. glaucum is similar to the core leptosporangiate ferns. The egg cell of D. glaucum was progressively isolated from the adjacent cells by forming a separation cavity and an egg envelope during maturation. However, during this process, a pore region consistently connected the egg and the ventral canal cell. Ultimately, a fertilization pore formed at the pore region when the egg matured. The nucleus produced conspicuous evaginations during later stages of egg development. The cytological features during oogenesis in D. glaucum more closely resembled the core leptosporangiate ferns rather than the basal leptosporangiate ferns (Lygodium and Osmunda), which possess no egg envelope or fertilization pore. The results provide new characteristics for interpreting the evolutionary history of the gleichenioid ferns.