Brown seaweed "Sargassum ilicifolium" contains alginate polysaccharide in their cell wall. Alginate is commonly used in a variety of applications in food industry as thickening, gelling agents, stabilizing agents, and food coating as well as coating drug for controlled delivery and cosmetic products. The experiments were conducted in SQU laboratories, the proximate analyses of Sargassum ilicifolium were determined and then the alginate extracted from raw seaweed and its purity and sulfate content were determined. In addition, the molecular weight, structure and thermal properties were characterized by using HPLC, FTIR and DSC, respectively. Also, the M/G ratio was estimated by using FTIR. In this study, yield and purity of the extracted alginate were 24.0 g/100 g dry seaweed and 97.0%, respectively. Moreover, the purity of the commercial alginate (99.3%) was very close from the purity of extracted one. The extracted alginate (2.5x10° Da) showed high molecular weight than commercial alginate (1.2x10° Da). Sulfate contents of commercial and extracted alginate were 2.13 and 4.47 g/100 g sample, respectively. The alginate in the extracted and commercial product were identified from the FTIR analysis considering high absorption peaks at 946.8 cm" and 1409 cm; and weak band at 2927 cm- and a broad band at 3264 cm. The guluronic acid (G) band was located at 1026 cm and mannuronic acid (M) at 1084 cm". The M/G ratios of bands were 0.69 and 0.70 for extracted and commercial alginate, respectively. The DSC thermogram for both alginates showed three shifts in baseline, which indicated glass transitions, and an endothermic peak indicated solids-melting. In the case of first and second glass transitions, it was observed that transitions decreased with the increase of moisture content. This was due to the plasticization of alginate molecules with water. However, in the case of commercial alginate, third transition decreased with the increase of moisture, while extracted alginate decreased after critical moisture content at 15 g/100 g sample. The change of heat capacity (4Cp) at glass transition decreased with the increase of moisture content. This was due to the decrease of amorphous domain in the presence of water. The solids-melting peak temperatures of commercial and extracted alginate were decreased with the increasing moisture content. The melting enthalpies (AH) of commercial and extracted alginate were increased with increasing moisture content. FTIR analysis showed that the vibration of the selected bands increased with the increase of moisture. However, the effect of moisture on the vibration was higher in commercial alginate as compared to the extracted one. The chemical and structural characteristics of extracted alginate brown seaweed observed in this study indicated closed similarity to the commercial alginate. Overall, the structural characteristics of extracted alginate were explored and documented. Therefore, the extracted alginate could be used in specific foods, pharmaceuticals and cosmetic products.