The crystallization properties of three regioregular poly(3-alkyl thiophene)s (P3ATs) are studied: poly(3-hexyl thiophene) (P3HT), poly(3-octyl thiophene) (P3OT), and poly(3-dodecyl thiophene) (P3DDT). The morphology of the isothermally crystallized samples is a whisker type. The values of the enthalpy of fusion of ideal crystals (DeltaH(u)(0)), determined from the melting point depression in the polymer-diluent system, are 99, 73.6, and 52 J/g for P3HT, P3OT, and P3DDT, respectively. The values of the equilibrium melting point (T-m(0)), determined from the Hoffman-Weeks extrapolation procedure, are 300, 230, and 180degreesC for P3HT, P3OT, and P3DDT, respectively. From the linear extrapolation of the P3AT data, the T-m(0) and DeltaH(u)(0) values of unsubstituted polythiophene are predicted to be 400degreesC and 139 J/g, respectively. The crystallization kinetics of these polymers are studied with differential scanning calorimetry, and the Avrami exponents vary between 0.6 and 1.4, indicating one-dimensional heterogeneous nucleation with linear growth. As the P3AT whiskers are produced from the chain-folding process, the Lauritzen-Hoffman growth rate theory is applied to analyze the temperature coefficient of the crystallization rate data. Graphical plots indicate a transition from regime I to regime II during isothermal crystallization for all the P3ATs studied. The fold surface energy and the work of chain folding calculated from the slopes of the graphical plots decrease with an increase in the number of carbon atoms of the side chain. The primary crystallization process of the side-chain crystallization is very fast and is attributed to the zipping effect of the main-chain crystals.