A polystyrene-block-oligo(2-tert-butylbutadiene)-block-polystyrene triblock copolymer was prepared and cyclized by end-to-end ring closure. Ring-shaped polystyrene-block-oligo (2-tert-butylbutadiene) was isolated from the coupling product via gel permeation chromatography (GPC) fractionation. The ring polymer was ozonized for decomposition of the oligo(2-tert-butylbutadiene) sequences selectively referring to the linear molecule. From GPC analysis of the decomposed products by ozonolysis, it was quantitatively confirmed that the fractionated product was 86% ring molecules. Single chain dimensions of the ring and linear molecules in a good solvent, benzene, and in a theta solvent, cyclohexane, were measured with small-angle neutron scattering. The ratios of the radii of gyration, R-g(ring)/R-g(linear), were 0.780 in benzene and 0.789 in cyclohexane. These were compared with theoretically predicted values.