A new cross-linkable regioregular poly(3-(5-hexenyl)thiophene) (P3HNT) was synthesized via the Grignard metathesis route for the purpose of stabilizing the film morphology in polymer photovoltaic cells. Bulk heterojunction photovoltaic cells with P3HNT and (6,6)-phenyl C-61 butyric acid methyl ester (PCBM) showed initial power conversion efficiency comparable to that. of cells with poly(3-hexylthiophene) (P3HT) due to the high crystallinity of the polymer in the films. P3HNT underwent cross-linking at the vinyl groups of the side chains upon a thermal treatment, which was confirmed by the insolubility of the films in organic solvents and by their thermal behavior. As a result, the formation of large aggregations of PCBM was prevented in P3HNT: PCBM films even after prolonged thermal annealing. In addition, the deterioration of the photoconversion performance was suppressed in the polymer photovoltaic cells compared to cells with noncross-linkable P3HT:PCBM.