The physicochemical properties of polyalkylthiophenes with various sidechain length were widely investigated in order to reveal the functions of alkyl sidechains in these polymers. The effects of the side-chains on the properties of polyalkylthiophenes can be explained by their steric hindrance and mobility. The steric hindrance of alkyl chain affected not only the polymerization mechanism of the monomers but also the redox potential, interchain distance, charge transport properties, and film morphology. The mobility of the side-chain influences the rate of dedoping, heat of transitions of polymers. The structure regio-regularity, stability of polarons/bipolarons, film morphologies, and interchain interactions determine the optical and electric properties of polyalkylthiophenes.