Three polystyrene-block-polyisoprene-block-polystyrenes (SIS) with different compositionswere prepared and cyclized by the coupling reaction between two end groups on the same molecules.Ring-shaped polystyrene-block-polyisoprenes (SI) were isolated from the coupling products by GPCfractionation. They were ozonized in order to decompose the polyisoprene blocks selectively referring tothe linear molecule. GPC analysis of the decomposed products obtained by ozonolysis quantitativelyconfirmed that the fractionated products include ca. 90% ring molecules. The morphological transition ofthe ring-shaped block copolymers depends on composition in essentially the same manner as that of thelinear triblock copolymers, whereas the domain spacings of SI ring-shaped block copolymers were allsmaller than those of the corresponding SIS linear triblock copolymers, which is attributed to loopedchain conformation.