Abstract: In 2008, photosynthetic performances of young seedlings of Sargassum horneri were investigated under various temperatures, irradiances and salinities by use of chlorophyll fluorescence measurements. Results showed that the highest survival temperature of young seedlings was 28℃ and the appropriate growth temperature ranged from 22 to 24℃. Under reduced solar irradiance, in situ fluorescence yields F_m′ decreased to 0.4, but recovered to 0.73 when the irradiance declined over time. Under full spectrum of solar irradiance, the F_m′of the seedlings decreased to 0.11 and recovered to 0.5 after 24 h under standard culture condition. Therefore young seedlings of this alga needed a shaded light climate for growth under greenhouse condition. Six hours treatments in 9~60 salinity of seawater and 1 h in fresh water did not affect the optimal chlorophyll fluorescence quantum yields (F_v/F_m), so epiphytic algae that were sensitive to osmotic changes could be removed by such treatments at fixed time intervals.