Ultrafast optical response in several polydiacetylenes (PDAs) with different side-groups and morphologies has been investigated by femtosecond absorption spectroscopy. Several nonlinear optical processes, i.e., excitonic absorption saturation, hole burning, Raman gain, inverse Raman scattering, optical Stark effect, and induced-phase modulation, have been observed and the mechanisms are discussed. The relaxation from photoexcited free excitons to self-trapped excitons (STEs) has been observed in both blue- and red-phase PDAs. The time constant is estimated as 100 - 150 fs. The decay of STEs in the blue-phase PDAs is nearly exponential with time constant of about 1.5 ps at 290 K and about 2.0 ps at 10 K. The decay curve in the red-phase PDAs substantially deviates from exponential function. It is fitted phenomenologically to biexponential functions with time constants of slightly shorter than 1 ps and about 5 - 10 ps. These two time constants correspond to relaxations to the ground state, respectively, from the unthermalized (hot) STE and from the thermalized STE.