Femtosecond transient absorption spectral measurements have been performed for sizeantrolled polydiacetylene microcrystals (30, 100, and 150 nm mean diameter) dispersed in water. Ultrafast relaxation processes within 5 ps after excitation were attributed to formation and decay of self-trapped exciton, which was independent on the size. A time-dependent spectral shift of transient bleaching, corresponding to the stationary excitonic absorption band, was observed in the time region from 10 ps to a few ns. It is explained as a thermalization of absorbed light energy within the microcrystal and the succeeding cooling by comparing transient absorption spectra w i t h temperature difference spectra of the ground state absorption. Size-dependent cooling dynamics of a 'hot' microcrystal was successfully demonstrated; the time constant increases w i t h the crystal size.