We measured the magnitude and the time response of the nonlinear refractive index (n2) of poly(3-hexylthiophene) by a femtosecond time-resolved interferometric technique. Decays of 320 fs and 3.0 ps were found. The n2 Value obtained was -6 X 10(-9) esu at 620 nm. Calculation of the dispersion of n2 showed that the negative value is consistent with that of a four-level model.