Three different forms of Undaria pinnatifida, the southern form (U. pinnatifida f. typica), the northern form (U. pinnatifida f. distans), and Samcheok form (recently cultivated strain), were examined for the contents and compositions of fucoidans. Fucoidans were extracted from the dried edible portions of three forms of U. pinnatifida in low pH condition, mainly by ethanol precipitation and CaCl2 treatment. It was shown that Samcheok form contains 1.8 and 3.5 times more fucoidans than the northern and the southern forms, respectively. The monosaccharide compositions of individual fucoidans were also varied. The fucoidans from the southern and the northern forms were shown to be composed of mainly fucose and galactose with the molar percentage ratios of 83.5%:16.5% and 87.4%:12.6%, respectively, indicating that these are F-type fucoidans. The fucoidan from Samcheok form, however, consisted of fucose (62.7%), galactose (32.9%), and small amount of glucose (4.4%). The results of this study showed that both amount and monosaccharide compositions of fucoidans are variable depending on U. pinnatifida forms.