Microphase-separated structures of a series of AB(6) block-graft copolymers were studied by TEM and SAXS. Ten copolymers with the same polystyrene (S) backbone and six polyisoprene (I) grafts on the average but with different graft chain lengths were carefully synthesized by living anionic polymerization, covering the range 0.21 <= phi(S) <= 0.90, where phi(S) denotes polystyrene compositions. From TEM observation of the AB(6) block-graft copolymers, it turns out to be clear that they show four microphase-separated structures, S-spheres, S-cylinders(S-prisms), alternative lamellae, and I-cylinders. Among them, for example, the samples with 0.54 <= phi(S) <= 0.58 shows prism structures whose cross sections of the S domains are close to hexagons, not circles, due to packing frustration of grafts. Composition dependence of morphologies of the present AB(6) block-graft copolymers reveals their phase diagram is extremely asymmetric with respect to phi(S) = 0.5. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 952-960