Poly (3-hexylthiophene) (P3HT) synthesized by oxidative polymerization was fractionated by molecular weight by using organic solvents. The fraction of higher average molecular weight gave higher regioregularity and conductivity. Composites of the P3HT fraction having the highest molecular weight were prepared by use of the following conducting particles as fillers: titanium carbide (TiC), indium tin oxide (ITO), and carbon black (CB). Temperature-conductivity profiles of the composites showed that the resistance change with PTC (positive temperature coefficient) effect was strongly influenced by the content and size of conducting particles and the molecular weight of P3HT. Although no significant PTC effect for P3HT-CB composite and little effect for P3HT-ITO composite system were observed, the P3HT-TiC composite containing TIC of 70-80 wt % showed an obvious PTC effect that brought the conductivity change by about four orders of magnitude near the glass transition temperature of P3HT. However, such a remarkable PTC effect was not observed for the P3HT-TiC composite prepared with the P3HT fraction of low-molecular weight. It was shown that a good PTC effect could be achieved by the composite consisting of the P3HT of high-molecular weight and the conducting particles of relatively large size.