The present invention provides an alginate oligosaccharide and its derivativeswith the degree of polymerization ranging from 2 to 22. The alginateoligosaccharide is composed of .beta.-D-mannuronic acid linked by 1,4glycosidic bondsand shown on Formula II. The derivatives with the reduced terminal in position1of carboxyl radical can be prepared by oxidative degradation. The presentinvention also provides a process for preparing the alginate oligosaccharideand itsderivatives, which includes the procedures than an alginate solution isreacted for 2to 6 h in an autoclave at pH 2-6 and the temperature of 100-120°C, andpH isadjusted to 7 after the reaction is stopped, after which the resultantoligosaccharideis oxidized in the presence of an oxidant to obtain an oxidative degradationproduct. The alginate oligosaccharide and its derivatives of the invention canbeused in the manufacture of a medicament for the prophylaxis and treatment ofADand diabetes.(see formula II)
