Nanowire lengths and length-to-width aspect ratios in regioregular poly(3-hexylthiophene) (P3HT) were simply controlled through changes in the solvent vapor pressure during solidification. It is demonstrated that the nanowires grew by rod-to-rod association, in which the molecular long axis of the P3HT chains appeared to be well-oriented parallel to the silicon substrate (Si/SiOx). The formation of the nanowires took place by one dimensional self-assembly, governed by pi-pi stacking of the P3HT units.