The (AB)(n) star copolymers (arm number n = 7-77; polyisoprene blocks = ca. 50 vol%) were synthesized by copolymerization of polystyrene (PS)-block-polyisoprene (PI) diblock anions with divinylbenzene (DVB) in benzene. The arm numbers were controlled by changing the initial concentration of the diblock anion or the feed ratio of DVB to the diblock anion end. The microphase-separated structures of (AB)(n) star copolymers were studied by means of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). It was found that the (AB)(n) star copolymers composed of symmetric arms (n < 77) formed lamellar structures. The periodic distances of (AB)(n) star copolymers (n < 77) were the same as those of the corresponding diblock arms.