| OBJECTIVE To isolate polysaccharides from Sargassum thunbergii and study their physicochemical properties, microstructures and anti-inflammation effects. METHODES Two polysaccharides (STP-I and STP-III) were purified using water extraction method, DEAE-Sepharose CL-6B and Sephacryl S-300 HR. The physicochemical properties and microstructures were studied using high-performance size-exclusion chromatography (HPSEC), gas chromatography (GC), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) analysis. Anti-inflammation activities of the polysaccharides were identified using real time quantitative PCR. RESULTS STP-I and STP-III were all homogeneous, molecular weights (Mw) were 29 and 350 kDa, and the percentage contents of total carbohydrates were more than 96%. TEM revealed further that STP-I was mesh-shaped, and STP-III was observed as linear microstructures with entangled chains. Due to the results of anti-inflammation activities of the polysaccharides, STP-III showed the higher inhibitory effects of TNF-α, IL-6 and COX-2 mRNA expressions in LPS-stimulated RAW 264.7 mouse macrophage cells than others, and the inhibitory effects were more than 95% at 150 μg . ml-1 concentrations. CONCLUSION The polysaccharides from S. thunbergii have obvious anti-inflammation effects and may serve as potential anti-inflammatory agents, and linear STP-III shows more noticeable anti-inflammation effects than the huge mesh-shaped STP-I.