The conjugated system of polymer has an important role on the photovoltaic properties of a polymer bulk heterojunction (BHJ) solar cell. In this work, we focused on the impact of π-conjugation length of poly(3-hexylthiophene) (P3HT) on the power conversion efficiency of P3HT:phenyl-C61-butyric-acid methyl ester (PCBM) BHJ solar cell. The π-conjugation length of P3HT is systematically controlled through partial bromination at the 4-position of thiophene ring, which elucidated the correlation between the degree of P3HT bromination and the photovoltaic properties of Br-P3HT:PCBM BHJ solar cells. The energy conversion efficiency of the unbrominated (pristine) P3HT:PCBM solar cell is 2.3%, whereas that of Br-P3HT:PCBM solar cell with approximately 7% bromination falls to 1.4%. The present results suggest that the π-conjugation length of about 14 thiophene rings is insufficient for achieving efficient charge transport.