Abstract: The crude polysaccharide was extracted from Porphyra yezoensis with hot water and was then purified by DE-23 and Sephadex G-200 column chromatography. A purified polysaccharides (PY1) was obtained, which contains little sulfate. Its homogeneity was identified with Sepharose 6B column chromatography. The average molecular weight was estimated to be 220 kD. The ultraviolet spectrum showed there is no proteins and nucleic acids in PY1. And the infrared spectrum indicated that there are special absorption of 3,6-anhydro_galactose and very little sulfated absorbance peaks. Galactose and its derivatives were the main constituents by gas chromatography. Through periodate oxidation, Smith degradation and 13C-NMR analysis, PY1 is mainly composed of agarobiose, and a little of its precusor exists as well.