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The spectra of the third‐order nonlinear optical susceptibility, χ(3), of poly(2,5‐thienylenevinylene) thin films were measured with third‐harmonic generation in the fundamental wavelength range of 1475–2100 nm. The effect of increasing the π‐conjugation length for χ(3) was investigated for both samples with varied extents of conversion and uniaxially stretched sample. The χ(3) values were intensively increased with the progress of conversion. Two χ(3) peaks at the fundamentals of 1600 and 1950 nm were found in the fully converted film. The largest χ(3) value reached 4.5×10-10 esu at 1950 nm, and the origin of the peak was ascribed to three‐photon resonance with the lowest π‐π* transition band. It was also found that the overall profile of the χ(3) spectrum drastically changed through uniaxial stretching. The reason for the observed change is discussed in terms of the difference in average π‐conjugation length.