Caulerpa lentillifera is green alga which is eaten and known as green sea caviar in Japan. After extraction with 70% ethanol, chloroform/methanol (1:1, v/v), aceton and hot water, polysaccharide was extracted by 24% KOH and collected as KOHP. KOHP became water-insoluble after neutralization with acetic acid. The results of neutral sugar composition analysis showed that the KOHP was mainly composed of xylose. Methylation analysis showed that the KOHP mainly consists of 1,3-linked xylose. The result of the optical rotation analysis indicated that the KOHP was composed of β-linked sugar. These results indicated that polysaccharide in C. lentillifera extracted by 24% KOH is relatively pure β-1,3-xylan.