Sargassum thunbergii is a characteristic plant in the rocky intertidal zone along the coast of China, Japan and Korea. This alga plays important roles in the maintenance of a healthy coastal ecosystem due to its large biomass and high productivity. Market demand for this alga is growing because of its high economic value in nutrition, pharmacy and aquaculture. Recently, the cultivation of Stichopus japonicus is rapidly developing in China. As its irreplaceable feed, utilization of S.thunbergii has been increasing, which has resulted in the severely damage of natural resources along the coast of China. Large-scale culture of this alga is the ultimate way for the conservation and sustainable utilization of the resource. Based on an ecological demography conducted from July 2009 to September 2009 in Wangfu Reef, Changdao, Yantai, the resource allocation trade-off between growth and reproduction of S.thunbergii was investigated. Three permanent quadrats at the same depth were set up, of which all thalli in a plot of 30 cm×30 cm was sampled once a week. Thalli inside the quadrats were randomly removed, bagged, and carried to the laboratory to estimate the relationships between maturation and the length, and the relationships between vegetative and reproductive tissue biomass. At the same time, thirty fertile and thirty sterile thalli were selected to investigate the effects of maturity on growth status，using repeated measures ANOVA. The results showed the following. Reproduction initiated at the beginning of July and peaked at the beginning of August (87% fertile thalli). The percentage of fertile thalli exhibited a statistically significant temporal variation. The population investigated in present study is markedly different from those growing on the coast of Pingtan Island, which results from the differences of photoperiod and seawater temperature. Temperature is the critical factor which induced S. thunbergii to translate from vegetative growth to reproduction. During reproduction period, the percentage of fertile individuals had a significant hierarchical variation, and the length of thalli played very important roles in the percentage of fertile thalli and reproduction effort. Thalli must achieve a critical size before maturation. This suggested the maturation was size-dependent. One underlying assumption in life-history theory is that the trade-off between growth and reproduction can explain this phenomenon: larger plants may take in more resources than smaller ones, which can generate a greater number of reproductive meristems per unit of vegetative biomass. At the peak of fertility, the individuals with greater vegetative biomass were characterized by higher reproductive output, but the different increment speeds between vegetative and reproductive tissue biomass were found. The energy accumulation and growth center varied with growth period indicating that the reproduction meristems growth relies on vegetative tissue and there is a trade-off between reproduction and vegetative growth. Because of partition of resource, senescence and rupture, after the peak of fertility, fertile thalli showed a pronounced negative growth rate while sterile individuals still exhibited a positive growth rate. These results indicate that a trade-off between growth and reproduction exists in the population of S. thunbergii. This life history trait is the result of adjusting to the stressful conditions of intertidal habitats.