Characteristic cylindrical microphase-separated structures have been found for three ABC star-shaped terpolymers. The samples are composed of polystyrene (S), polyisoprene (I), and poly(2-vinylpyridine) (P); their volume ratios of I:S:P are about 1:1:0.7, 1:1:1.2, and 1:1:1.9, respectively, three components being connected at one branch point. From three-dimensional morphological observation by TEM combined with electron computerized tomography, a terpolymer with volume ratio of 1:1:0.7 shows honeycomb-type microdomain structure, i.e., (6(3)), having three kinds of hexagonal cylinders arranged hexagonally with junction points being arrayed one-dimensionally at the intersection of three domains, while others show characteristic cylindrical structures with mainly 4-fold, (4.8(2)), and 6-fold, (4.6.12), symmetry. It has been clarified, however, that many defects exist in the regular microdomain structures of all three terpolymers.