The effect of alkyl side chain length on the molecular ordering and electrical properties of regioregular poly(3-alkylthi-ophene) (P3AT)-based field-effect transistors (FETs) was investigated using P3ATs with various alkyl side chain lengths (–(CH 2) n CH 3 , n = 3, 5, and 7) as active materials. The inner structures and surface morphologies of the P3ATs thin films fabricated on an insulator substrate were characterized, and the electrical properties of FETs based on these films were correlated with the structure and alkyl chain length of the P3ATs. The FET based on poly(3-butylthiophene), which has the shortest alkyl side chains among the three P3ATs considered, showed the highest field-effect mobility, possibly because the film formed from molecules with short butyl side chains had a higher density of p-stacked ordered structures in the charge transport region.