Abstract: Bangia sp. was collected from the cultivated population in Putian (Fujian province, China). It had a typical biphasic life history with alternation between a gametophytic phase (thallus) and a sporophytic phase (conchocelis). Both sexual and asexual reproductive modes presented in this Bangia population. The ultrastructural characteristics of the gametophytic vegetative cells, archeospores, carpospores, vegetative conchocelis cells and conchosporangial cells were investigated. The results indicated that the growth and development of the gametophytic phase are related to the reproductive unit formation. There were plenty of Floridean starch granules and 7-42 thylakoids containing phycobilisomes in the vegetative gamtophytic cells. During archeospore formation, floridean starch granules broke down and few thylakoids irregularly arranged were observed. Mitochondria were spherical or elliptical and were situated peripherally in the cell. Carpospores were naked and could make ameboid movements as they were just released. Floridean starch granules were distinct and disintegrated. The numbers of phycobilisome, endoplasmic reticulum, ribosome, vacuole, fibrillar visicle and core visicle increased in carpospores. Mitochondria were located peripherally or around the nucleolus and most were dumbbell-shaped. There were most 8 thylakoids in carpospores and the peripheral thylakoids were not present. In conclusion, both phases contain stellate chromotophore and pyrenoid, showing the Bangiophycidae characteristics. The gametophytic phase lacks plugged pit-connections and peripheral thylakoids, indicating the Bangiophyccidae characteristics, while the vegetative conchocelis has plugged pit-connections and peripheral thylakoids, showing the Florideophycidae characteristics. The conchosporangial branches have plugged pit-connection and lack peripheral thylakoids and may serve as a link between the two phases.