Single crystals of poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran vapor annealing and controlling solvent evaporation, respectively. The morphology and structure of the single crystals are characterized using optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that in P3HT single crystals, the molecules are packed with pi-pi stacking direction perpendicular to the length axis of the crystals and main chains parallel to the substrate, whereas in P3OT single crystals, the molecules are packed with pi-pi stacking direction parallel to the length axis of the crystal and main chains parallel to the substrate. In the field effect transistors, the current flow is parallel to the length axis of the single crystals, and the mobility is 1.57 x 10(-3) cm(2)/Vs for a P3HT single crystal and 0.62 cm(2)/Vs for a P3OT single crystal. The single crystals of P3HT and P3OT showed high anisotropic electrical properties. The influences of molecular conformation and alkyl chain length on the electrical properties of P3ATs are discussed.