Abstract: To understand the sexual reproduction and paedomorphosis of Sargassum henslouianum at different temperatures (10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃), we observed the reproduction process of S.henslouianum, survival rate and development of zygotesfromMarch to May, 2014. Results show that 3~5 eggs were released from each female conceptacle. By nuclear division, freshly released eggs with single nucleus turned into eight-nucleus eggs. After being fertilized, the eggs developed into multicellular structures in 24 h without dormancy. The optimum temperatures for ovulation was 20 ℃ with survival rate of 89.0% and for development of seedlings was 25 ℃.The first and second buds of branches were observed after being cultured for 15 d and 18 d, respectively, and the average length of seedlings was 2~3 mm.