Well-defined [poly(methyl methacrylate)]2poly(3-hexylthiophene) miktoarm star copolymers (PMMA2P3HT) were successfully synthesized via anionic coupling reaction. P3HT with two bromine groups at one chain end (P3HT-Br2) was synthesized by Williamson reaction between excess amount of tris(bromomethyl)benzene and hydroxyl-terminated P3HT. From anionic coupling reaction between living PMMA anions and P3HT-Br2, we prepared a series of PMMA2P3HTs having narrow molecular weight distribution (polydispersity index < 1.21) with various block compositions. While most P3HT-containing linear rod–coil block copolymers show only fibril structure, PMMA2P3HT shows conventional block copolymer self-assembled structures. Namely, spherical, hexagonally packed cylindrical, and lamellar microdomains including fibril structure were formed, confirmed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), depending on the weight fraction of P3HT (wP3HT). Even at a wP3HT = 0.72, lamellar microdomains were observed because of the curvature effect resulting from miktoarm architecture at the interface between two blocks. The result implies that the macromolecular architecture is one of the important factors for adjusting self-assembled morphology of P3HT-containing block copolymers. Moreover, the melting temperature of P3HT in PMMA2P3HT having lamellar or cylindrical morphology does not decrease compared with neat P3HT homopolymer, which means that the rod/rod interaction of P3HT was well-maintained under miktoarm architecture.