The photodegradation mechanism in coumarin laser dye solutions is investigated at 77K by the direct detection of free radicals using ESR technique and of products using spectrometric identification after degradation under UV flashes. As a result, the following degradation mechanism is proposed: the energy is transferred from a dye molucule（T1）to an O2 molecule. The excited O2 molecule oxidizes a free radical in the solvent. The dye of degradation is caused mainly by O2 oxidation.