The free radicals formed from gamma-irradiated aqueous solutions of thymine have been investigated by the spin-trapping method. The solutions containing thymine and spin-trapping reagent tert-nitrosobutane in H2O or D2O were degassed by repeated freezing and were gamma-irradiated in 60Co source at room temperature to a total dose of 0.13 Mrad. Sodium formate was used as a scavenger for H and OH to identify the radical formed by hydrated electron. The transient thymine radicals were spin-trapped by tert-nitrosobutane and detected by electron spin resonance spectrometer. Three thymine radicals were observed in our experiment.