Styrene(S)-isoprene(I) star-shaped block copolymers of the (SI)4 and (SI)12 types were prepared by an anionic polymerization technique. Linear SI arm molecules were prepared with sec-butyllithium as an initiator in benzene, followed by a coupling reaction with the 4-functional linking agent 1,2-bis(methyl-dichlorosilyl)ethane and the 12-functional 1,1,2,2-tetrakis(2-(trichlorosilyl)ethyl)silane, respectively, under vacuum. The star-shaped block copolymers were isolated by precipitational fractionation from as-polymerized crude products using benzene-methanol mixtures. They were confirmed to have the designed molecular structures by careful characterization. Using a transmission electron microscope it was observed that the microphase-separated structures of solvent-cast films of the samples are polystyrene (PS) cylinders in a polyisoprene (PI) matrix, alternating lamellar structures and PI cylinders in a PS matrix for 4-armed star-shaped block copolymers when the values of the volume fraction of PS, phi(s), are 0.23, 0.47 and 0.78, respectively, and lamellar structures for the 12-armed star-shaped block copolymers when the phi(s) values are approximately 0.5. Small-angle X-ray scattering studies revealed that lamellar domain spacings of the star-shaped block copolymers are essentially the same as those of the arm molecules, that is, linear diblock copolymers, irrespective of the number of arms.