D-enzyme (EC 2.4.1.25) is believed to be involved in starch metabolism, but the function in vivo is not known. In order to investigate the role of D-enzyme, several approaches have been under taken. Transgenic potato plants with dramatically reduced D-enzyme activity were obtained by introducing sense and antisense D-enzyme cDNA sequences with the appropriate promoter sequence. The tubers from these plants sprouted later and the growth of sprouts was slower than the wild type. However, no significant difference was found in starch produced in tubers, either in its quantity or quality. These results seems to suggest the role of D-enzyme in starch degradation rather than starch synthesis. The analysis of the action of D-enzyme on amylose and amylopectin revealed that the enzyme can catalyze the cyclization reaction and produce cycloamylose with DP larger than 17. Cycloamylose produced are highly soluble in water and can incorporate various compounds in their helical cavities, thus expected to find exploitation in food and pharmaceutical industries.