Structure of fucoidan from sea cucumber Acaudina molpadioides (Am-FUC) was investigated in this study. Low molecular weight polysaccharide of Am-FUC (Am-LMW) was prepared by enzymatic degradation and assayed by nuclear magnetic resonance. According to the structure of Am-LMW, and referring to previously reported information of Am-FUC oligosaccharides, the structure of Am-FUC was elucidated as [→3-α-l-Fucp-1→3-α-l-Fucp2,4(OSO3(-))-1→3-α-l-Fucp-1→3-α-l-Fucp2(OSO3(-))-1→]n. Compared to other investigated fucoidans, which also consist of tetrafucose repeating units with 1→3-linkages, Am-FUC contains a novel repeating unit distinctive in sulphation pattern.