Soluble conjugated polymers, poly (3-alkylthiophene)s were prepared by chemically oxidative polymerization. The polymers had no absorption at wavelengths longer than 600 nm, indicating that the polymers were suitable for waveguide structures. Dispersion of refractive index (real and imaginary parts) was calculated from the reflectance data using the Kramers-Kronig relation. The dispersion agreed with the result obtained by ellipsometry. Third order optical nonlinearity, χ(3)(−3ω; ω, ω, ω), of the polymers was determined by optical third harmonic generation (THG). Enhancement of the χ(3) by three photon absorption was observed when a beam of 1.54 μm was used as a fundamental. Also, light propagation properties were investigated by using slab waveguides of the polymers.