The phase behavior of 18-arm star-shaped polystyrene-block-poly(methyl methacrylate) copolymers ((PS-b-PMMA)18) with various volume fractions of PMMA block (fPMMA) was investigated by transmission electron microscopy and small-angle X-ray scattering. (PS-b-PMMA)18 was synthesized by atom transfer radical polymerization from α-cyclodextrin (α-CD) having 18 functional groups for the initiation. We also prepared the corresponding linear PS-b-PMMAs by cutting the ester groups connecting α-CD and PS chains in (PS-b-PMMA)18 through the hydrolysis. The microdomains of (PS-b-PMMA)18 changed from body-centered-cubic spheres (BCC), hexagonally packed cylinders (HEX), perforated lamellae (PL), and lamellae (LAM), with increasing fPMMA from 0.3 to 0.8. Interestingly, (PS-b-PMMA)18 with fPMMA of 0.77 showed highly asymmetric lamellar microdomains, while the corresponding linear PS-b-PMMA with the same volume fraction should not have lamellar microdomains. Thus, the microdomains are highly affected by the molecular architecture of block copolymer. The experimental results are discussed with the prediction based on the self-consistent mean-field theory.