Typical conductive polymers of poly(3-alkylthiophenes) were synthesized by oxidative polymerization. Alkyl side chains were CnH2n+1, with n=4, 6, 8, 12. The regioregularity with the HT linkage was larger than 99% based on NMR analysis. Completely regioregular head-to-tail (HT) poly(3-alkylthiophenes) were obtained. We have evaluated the effect of side chain size on the thermoelectric properties of Seebeck coefficient and electrical conductivity. The results were as follows: 1) Seebeck coefficient decreased with an increasing electrical conductivity. 2) High Seebeck coefficient > 1mV/K was observed at low electrical conductivity < 10(-2)S/cm. 3) The small side chain caused the higher electrical conductivity in the range of electrical conductivity < 10(-1)S/cm.