Ultrafast optical response in polydiacetylenes and polythiophense was investigated by femtosecond time‐resolved absorption spectroscopy. Several nonlinear optical processes, i.e. hole burning, Raman gain, and induced‐frequency shift, were observed and the third‐order nonlinear susceptibilities were determined. The ultrafast relaxation dynamics following after the photoexcitation was investigated and the formation and relaxation of self‐trapped exciton was observed. A model of the relaxation processes of excitons in conjugated polymers is proposed. The short life of STE and difference between fluorescent and nonfluorescent polymers are explained.