Abstract: Nuclear division of eggs and cell division of zygote of Sargassum cinereum were observed by tissue culture in vitro. The effects of various salinity values (15‰, 22‰, 26‰, 30‰, 34‰, 38‰, and 42‰) on egg release and early development of S. cinereum germlings were studied under laboratory conditions. The results indicated that the algal eggs could be released normally in the salinity range studied. The eggs were released concentratedly and rapidly in the salinity range of 22‰~34‰; however, at 15‰, 38‰, or 42‰, concentrated egg release was delayed. More eggs were detected at salinity over 30‰ than below 26‰. A freshly released unfertilized egg from the nest hole had only one nucleus in the center, which was then divided into eight daughter nuclei within about 4 hours; finally only one combined with sperm nucleus, and changed into zygote with a large nucleus in the center. The first two cell divisions of zygote were transverse, and the subsequent two were all longitudinal. Four to five hours after fertilization were needed for the four cell division, then zygote cleavage was speeded up and cell number was increased rapidly, and finally a multicellular germling was formed within about 22 hours. The rhizoid grew prior to thallus in the next day, and then thallus length increased gradually. Salinity significantly affected the early development of S. cinereum germlings. Rhizoid grew normally in salinity range of 22‰~38‰; however, higher or lower salinity inhibited its forming and elongation, and the inhibition action at 42‰ was stronger than that at 15‰. In addition, the suitable salinity for growth was 26‰~38‰ for S. cinereum thallus. In conclusion, the suitable salinity was 26‰~34‰, and the optimal salinity was 34‰ for early development of S. cinereum germlings.