In this article, the uniaxial alignment of poly(3-hexylthiophene) (P3HT) nanofibrils with a π-π stacking growth direction in which P3HT chains adopt a flat-on conformation was obtained by solvent directional evaporation using a glass cover slide and a poly(dimethylsiloxane) (PDMS) sheet to press the P3HT film in a carbon disulfide (CS(2)) atmosphere. By controlling the CS(2) vapor pressure during the film-forming process, we got a well-oriented P3HT film whose order parameter reached as high as 0.97. The orientation of the film was induced by the crystallization nucleation of P3HT and the directional evaporation of the solvent. Under a CS(2) vapor atmosphere, P3HT crystals preferred to adopt the form II modification, which started by nucleation. Owing to the solvent directional evaporation from the center to the margin, P3HT at the center of the sample would precipitate first to induce nucleation. Then the peripheral P3HT would directly diffuse, precipitate, and then adhere to the nucleus to form the uniaxial alignment of P3HT nanofibrils along the direction of solvent evaporation. Furthermore, in the P3HT nanofibrils, the π-π stacking direction of P3HT lamellae was parallel to the crystal growth direction, which would provide an effective path for charge transport.