Drastic change of properties of conducting polymer by substitution of side group has been demonstrated utilizing poly(3-alkylthiophene) as s typical example. Electrical (conductivity, mobility) and optical (absorption, luminescence) properties of poly(3-alkylthiophene) were strongly dependent on alkyl chain length, temperature and ambient conditions. These results can be explained by taking account fluctuation of electronic band scheme and change in inter-chain interaction which are dependent on the alkyl chain length and its conformation. Properties of mixture of poly(3-alkylthiophene)s with different alkyl chain length were also discussed. Doping characteristics of poly(3-alkylthiophene) dependent on alkyl chain length were discussed in terms of modification of crystal structure, location of dopants, and polymer-dopant interaction. Especially, unique doping effect of C60 in poly(3-alkylthiophene) such as photo-induced charge transfer was also observed.