Magnetic field effects (MFEs) on photoelectrochemical reaction of electrodes modified with nanowires consisting of regioregular poly(3-hexylthiophene) (P3HT) and disordered P3HT films without and with anneal treatments were examined. The MFEs on the photocurrents were observed in three modified electrodes. The magnitude of the MFEs increased with the following order of modified electrodes: P3HT nanowires, P3HT films with anneal treatment, and P3HT films without anneal treatment. The difference of the MFEs is most likely attributable to the facility of hole hopping between the adjacent thiophenes in the P3HT structures.