The present invention relates to a bioavailable fucoidan characterized in that the bioavailable fucoidan has (a) 20-40% of the content of sulfate group; (b) 1,000-30,000 Da of the weight average molecular weight; (c) 1-6 of polydispersity; and (d) 0-200 centi-poise of viscosity. Since the low molecular weighted fucoidan of the present invention has higher content of sulfate groups, low weight average molecular weight of less than 30,000 Da, low polydispersity and viscosity, and high water-solubility, its absorption rate in the body is greatly increased resulting in high improvement in its bioavailability. Accordingly, it would be appreciated that the present low molecular weight fucoidan sufficiently exerts its unique physiological activities in the body (inhibition of cancer cell proliferation, anti-oxidative activities, suppression of blood glucose level, anticoagulant effects, suppression of triglyceride and cholesterol level, and improvement in the therapeutic effect to gastric ulcer). In addition, the improved physical properties of bioavailable fucoidans of this invention ensure to overcome problems owing to non- homogeneous quality in conventional fucoidan products.