We have demonstrated a facile synthetic route for well-defined poly(3-hexylthiophene)-block-poly(methyl methacrylate) copolymer (P3HT-b-PMMA) by anionic coupling reaction. For successful coupling reaction, newly generated anions should be more stable (less reactive) than initial anions of reactants. In this study, we chose α-phenyl acrylate (PA)-capped P3HTs for successful coupling with living PMMA anions because the anions at PA group are more stable than living PMMA anions. We found that all of the PA groups located at the end of P3HT were completely coupled with living PMMA anions having slightly excess amount (1.5 equiv relative to PA-capped P3HT). The unreactive PMMA homopolymers in crude product were completely removed by using column chromatography, resulting in narrow molecular weight distribution of pure P3HT-b-PMMA. The optical property and thin film morphology of the P3HT-b-PMMA were investigated by using UV-vis spectra and atomic force microscopy, respectively.