The structures and properties of polysaccharide fromEucheuma gelxtinae were studied, and according to the results of experiment, the gelation mechanism was deduced.The polysaccharide gel in the presence of K⁺, Rb⁺, Gs⁺, Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, Ba⁺⁺ and NH₄⁺ forms gel which has higher strength than the original. The strength of gel increases with concentration of adding salt, and it attains the maximum value as the ratio of cation number to galactose residue is 1:2, and then starts decreasing The polysaccharide reacted with compounds contain two aminogroups such as urea, alkaline amino-acid and ethylenediamine to form higher strength gel.The polysaccharide also reacted with gelatin to form complex which was a high strength elastomer. The complex could react with the formaldehyde to form a very high strength gel. Its strength was about twice as that of the original complex gel.The scanning electron microscope revealed that the shape and aggregating compactness of section of gel after freeze-drying agree with the order of dehydrating tendency. From above results, it may be considered that the reaction mechanism of cation on the polysaccharide was different from that of diamine and gelatin.The research into various polysaccharide gel by scanning electron micrscope. indicated that the polysaccharide gel was like the sparse twisting fiber cluster and aggregated each other in a same direction; the polysaccharide-KCI was like the untwisting fiber cluster; th and aggregated in a same direction; the polysaccharide ethylene diamine was like some chains aggregating in a same direction; and polysaccharide-gelatin was like sponge.According to above evidence, the gelation mechanism was deduced as follows:1) When the temperature of polysaccharide sol-gel lows, the Brownian movement of molecules is weakened. In addition, the polysaccharide molecule exl (?)bits asymmetry because it is alternately linear comprised by galactose residues and 3,6-anhydro-D-galactose residues, with repeated structure. The neighbor molecule. segments were held together regularly to form a three dimentional framework by hydrogen bonds.2) The polysaccharide gel particles, in presence of cation (K⁺), dehydrated due to the hydration of cation. In addition, the negatively charged gel particles were neutraIized by cation. Then the macromolecuIe gel particles further approaches to each other, and regularly associated to develop a cohesive network by cation "salt bridges".3) In case of adding the compound which contain two amino-groups such as ethylenediamine, the amino-groups of the compound reacted with sulfate groups in the poiysaccharide to form amino-group salts, crosslinking to network structure.4) In case that gelatin was added, multiple amino-groups in gelation reacted with sulfate groups in the polysaccharide to form amino salts and develop a cohesive network involved both macromolecules.