The third order nonlinear responses for soluble one-dimensional ϕ-conjugated polymers, poly(3-alkylthiophene)s were investigated. Uniaxial stretching of the polymer film was carried out to align the polymer backbone. The stretched film showed large dichroism and crystallinity. Third order nonlinear susceptibility, χ(3) 1111 of the stretched film was determined by optical third harmonic generation. A large enhancement of χ(3) due to the main chain alignment was observed.