The relaxation dynamics of photoexcitations in a polydi-acetylene’ (poly-3BCMU) film was investigated by femtosecond time-resolved absorption spectroscopy. The spectral change due to several nonlinear optical processes, i.e., hole burning, Raman gain, and dynamic Stark effect, were observed. The difference absorption spectra of singlet and triplet excitons minus ground state were measured. The photoexcited singlet excitons relaxed to the self-trapped state with the time constant of 150 ± 50 fs. The lifetime of the self-trapped excitons was 2.0 ± 0.1 ps at 10 K and 1.5 ± 0.2 ps at 290 K. The lifetime of the triplet excitons was much longer than several tens of picoseconds and was consistent with the lifetime of 18 us obtained by nanosecond spectroscopy.