Microphase-separated structures of a series of AB(n) block-graft copolymers were studied by transmission electron microscopy (TEM) coupled with small-angle X-ray scattering (SAXS). Five block-graft copolymers composed of the same polystyrene (S) backbone having polyisoprene (I) grafts with different chain lengths, wherein the numbers of grafts are almost constant (38 on average), were synthesized by living anionic polymerizations, resulting in coverage of the polystyrene composition range 0.32 <= phi(S) <= 0.91. It was confirmed from the TEM observation that three low phi(S) samples with phi(S) of 0.32, 0.37, and 0.39 show complex spherical structures, while the sample with an phi(S) of 0.57 reveals a hexagonal assembly of hexagonal rods of S in an I matrix, and the sample with phi(S) of 0.91 exhibits inverse morphologies having rods of I in an S matrix. From detailed TEM and SAXS investigation combined with simulations, it has been found that the spherical structures formed from the samples with phi(S) of 0.32 and 0.37 are both confirmed to have the structural feature of the Frank-Kasper A15 phase, which possesses the structural symmetry of the Pm (3) over barn space group.