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 similar to 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 similar to 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.