Abstract: The current study explored features of maturated parental materials, embryonic development and young sporophyte germlings, and investigated effects of environmental factors, such as water temperature, salinity, light intensity and photoperiod, on the early development of young sporophyte germlings of Sargassum horneri collected from Mazu Ao, Nanji Island, Zhejiang Province, from 2011 to 2013. The results showed that the peak period for natural reproduction of Sargassum horneri was from late May to early March in each year in the coastal area of southern Zhejiang with water temperature under 13—18℃. The suitable temperature for the development of the zygotes was 10—20℃, and suitable salinity was 20—30, in which fertilized zygotes synchronously developed into young sporophyte germlings with strong rhizoids after about 48h incubations. In addition, the proper environmental factors for the growth and development of young sporophytes with rhizoid are under the temperature of 20℃, salinity of 20—30 ppt, light intensity of 4500lx, and photoperiod of 12—18h/d. In the early stage, the flowing water and air inflation were beneficial to indoor cultivations of the young sporophytes that developed to seedlings with 2—3 leaves after 18—20 days of cultivation. Hard cement bricks with rough surface were suitable for attachment and growth of young sporophytes inSargassum horneri.