Optical pumping effect on the third order optical nonlinearity of poly(3-dodecylthiophene) was investigated by 2 beam degenerate four wave mixing technique using low repetition rate femtosecond Ti:S amplifier. Four wave mixing signal from the sample was significantly enhanced by optical pumping at 400 nm. Femtosecond time resolved studies on four wave mixing and excited state dynamics of the entitled compound were carried out to elucidate the mechanism of enhancement. Exciton formed by optical pumping in one dimensional polymer backbone might presumably play an important rule on the enhancement of four wave mixing amplitude.