Abstract: During breeding season (in April), three types of gonadal structure of triploid crucian carp, produced by crossing F3-F8, allotetraploids (♂) of red crucian carp (Carassium auratus red var.) (♀) × common carp (Cyprinus carpio L.) (♂) with Japanese crucian carp (Carassius auratus Cavieri T.et S) (♀), were found and observed under light and electron microscope.The first type was the testis that consisted of many lobules in which there were numerous spermatides.Some degenerated spermatides were found and no mature spermatozoon was observed.The second type was the ovary-like gonad consisting of many nests of small undeveloped cells and a few small growing oocytes as well as enlarged and degenerated oocytes.The third type was the fat tissue, in which case, only two strips of fat tissue located on the gonadal places, neither testis nor ovary being observed.