Abstract: Sargassum horneri is one of the main components in the subtidal seaweed flora along the Chinese warmer water coast. With the changes of global climate and the increases of human activities, the once abundant algal biomass of S.horneri along the coast of Nanji Island (27°27′N, 121°05′E) has been reduced in recent years. In order to understand this changes, in situ observation of field individual plants were performed during 2006~2007 at four field sites distributed at intertidal zones. It was found that, (1) field population was mainly composed of the annual sporophytes derived from zygote via sexual reproduction, while the biannual plant was only about 5% of the total population; (2) the optimal temperature for growth and reproduction was 11~16℃. Sexual reproduction peaked at the temperatures of 16~20℃. Sporophytes of S.horneri are tolerant of a wide spectrum of irradiance, and are thus one of the ideal candidates in reconstruction of artificial seaweed beds.