The crystallization and melting behavior of poly(3-butylthiophene) (P3BT), poly(3-octylthiophene) (P3OT), and poly(3-dodecylthiophene) (P3DDT) were studied. The equilibrium melting temperatures (Tm0) of these polymers were measured and resulted equal to 321, 230, and 175 °C for P3BT, P3OT, and P3DDT, respectively. The crystallization kinetics was evaluated by the Avrami equation: it proceeds by heterogeneous nucleation with one-dimensional linear growth (n = 1.1−1.8). The effects of the length of the alkyl side chain and of the undercooling were investigated. Phase I, Phase II, and a nematic mesophase were detected in P3BT and P3DDT. In P3BT, Phase I was differentiable from the mesophase on the basis of wide-angle X-ray scattering (WAXS) patterns. This study gives insight, in particular, on the crystallization and melting behavior of P3BT, a polymer not yet well studied. WAXS, small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC) were employed as experimental techniques.