We synthesized, via anionic coupling reaction, poly(3-dodecylthiophene)- block-poly(methyl methacrylate) copolymers (P3DDT-b-PMMA) having narrow molecular weight distribution and several block compositions. P3DDT was chosen because of moderate rod/rod interaction compared with a weak interaction of poly(3-(2′-ethyl)hexylthiophene) (P3EHT) or a strong interaction of poly(3-hexylthiophene) (P3HT). The moderate rod/rod interaction of P3DDT enables us to investigate final morphologies affected by crystallization arising from the rod/rod interaction of P3DDT or the microphase separation between P3DDT and PMMA blocks. When the weight fraction (w P3DDT) of P3DDT block of P3DDT-b-PMMAs was smaller than ∼0.6, various microdomains such as body-centered-cubic spheres, hexagonally packed cylinders, and lamellae were observed similar to those reported in conventional coil-coil type block copolymers. Interestingly, these microdomains were maintained even after P3DDT blocks were crystallized, indicating that P3DDT crystals were successfully confined within P3DDT microdomains. On the other hand, when w P3DDT was high (e.g., w P3DDT ∼ 0.76), the rod/rod interaction became dominant over microphase separation between two blocks. As a result, only fibril structure was found after the crystallization of P3DDT block.