Sultanate of Oman is rich in many natural resources including seaweeds. Seaweeds can be used as food or feedstock for industrial processes. Sulfated polysaccharides are one high value functional compounds extracted from seaweeds. In Oman, seaweeds have not yet been utilized. Sulfated polysaccharides, especially fucoidans have been suggested to have various biological activities against many diseases, in particularly anti-cancer against gastric cancer. Stomach cancer is the second most frequent cancer in Omani males in 2013 and the third most common cause of cancer death among men worldwide in 2012. This study aimed to extract the fucoidans from species Sargassum ilicifolum, characterize it chemically and investigate its antioxidant potential properties in a rat model of nitrosamine (NA)-induced gastric cancer pathogenesis. Sargassum ilicifolum sundried powder was analyzed to find out its chemical composition and to extract fucoidans polysaccharides in two fractions A and B, using calcium chloride & diluted hydrochloric acid extraction method. The chemical characterizations of extracted fucoidans were elucidated using Spectrophotometer techniques to evaluate the total sugar and sulfate content, High Performance Liquid Chromatography to identify the molecular weights and Fourier Transform InfraRed technique to describe the fucoidans structure. Four rats groups of control group, (NA) injected group, fucoiadans extract supplemented group and NA -injected with fucoidans extract treatment group, were used to evaluate the antioxidant potential of fucoidans against NA-induced oxidative tress and cancer pathogenesis. Extracted fucoidans showed that the carbohydrate was the highest constituent of Sargassum ilicifolum with percentage of about 50% in the dry basis. In addition, the findings of study revealed that extracted fucoidans contained about 28% as total sugar in the both extracted fractions A and B, 13.6 £ 2.3% in fraction A and 14.6 + 1.3% in fraction B of sulfate content located at C-2 and C-4, and 22.35 € 1.77% in fraction A and 15.55 + 0.31% in fraction B of fucose value. The different constitute of fraction A and B possessed molecular weights: 5513, 640, 64.9 and 3.25 kDa, that gave fucoidans protective effect against the chemically induced oxidative stress in rat stomach cell. The protective effect against oxidative stress in rat stomach cells injected with (NA) and treated with fucoidans was evidenced by improvement in the structure of mitochondria organelle with decrease in its dense matrix. In conclusion, the results of study suggested that fucoidans extracted from Sargassum ilicifolum has the potential to inhibit the chemically-induced oxidative stress in rat stomach cells, which may attributed to have high content of sulfate and variable MWs. Further work is needed to produce more pure fucoidans extract through fractionation to get compound with single molecular weight and enhanced chemical characteristics. Further scientific researches are also required to study the dose-dependent effect of fucoidans, molecular mechanism underlying the fucoidans protective effects against NA-mediated stomach cancer pathogenesis and any cell toxicity associated with fucoidans which is necessary to determine its safe limit as therapeutic agent.