This study presents a structural analysis of regioregular poly(3-hexylthiophene) (P3HT) based on electron diffraction from epitaxied thin films. Epitaxial orientation of the hexane fraction of P3HT was performed by slow rate directional solidification in 1,3,5-trichlorobenzene leading to highly oriented and crystalline P3HT films with different contact planes. Representative electron diffraction patterns corresponding to different zone axes were obtained by the rotation-tilt electron diffraction method. A trial-and-error method based on molecular modeling and calculation of the electron diffraction patterns for the different zone axes was used to determine the crystal structure of P3HT. The unit cell is monoclinic with space group P2(1)/c and two chains per cell (a = 1.60 nm, b = 0.78 nm, c = 0.78 nm and gamma = 86.5 deg). The stacking period of successive polythiophene backbones along the h axis is 0.39 nm but short interplanar distances of 0.34 nm are observed because the conjugated polythiophene backbones are tilted to the h axis. The n-hexyl side groups crystallize in an orthogonal subcell with parameters a(s) = 0.7 nm and b(s) = 0.78 nm. The present structural model highlights the essential role of the linear side chain crystallization on the supra-macromolecular packing of "hairy-rod" polymers like P3HT.