A kind of alfalfa seeds was irradiated by 1, 2, 3, 4 and 5 kGy at a dose rate of 6.288 kGy·h-1 in a self-shielded irradiator of 137Cs gamma rays. The EPR spectra, which were measured subsequently between 0.3401 and 0.3501 T, showed that there was a direct proportional relationship between the EPR signal strength of free radi- cals produced by gamma irradiation in the alfalfa seeds and absorbed dose. The first derivative EPR spectra of the al- falfa seeds were very clear and easy to identify. However, the EPR signal strength of the peak-to-peak amplitude de- creased rapidly and most of them decayed beyond 50% within 3 days after the seeds were irradiated. It tended to sta- bilize after half a month since the seeds were irradiated. The differences of the EPR signal strength between the irra- diated and unirradiated alfalfa seeds still remained. All seeds were stored at ambient temperature for more than 3 months. Therefore, using EPR spectrometry technique to measure free radicals in alfalfa seeds as a means to deter- mine whether the seeds have been irradiated or not is feasible, relatively fast and simple. The kind of alfalfa seeds was irradiated by 1, 2, 3, 4 and 5 kGy at a dose rate of 6.288 kGy · h-1 in a self-shielded irradiator of 137Cs gamma rays. The EPR spectra, which were measured between 0.3401 and 0.3501 T, showed that there was a direct proportional relationship between the EPR signal strength of free radi-cals produced by gamma irradiation in the alfaalfa seeds and absorbed dose. The first derivative EPR spectra of the alfa falfa seeds were very clear and easy To identify. However, the EPR signal strength of the peak-to-peak amplitude de- creased rapidly and most of them decayed beyond 50% within 3 days after the seeds were irradiated. It tended to sta- bilize after half a month since the The differences of the EPR signal strength between the irra- diated and unirradiated alfalfa seeds are still. All seeds were stored at ambient temperature for more than 3 months. Using EPR spectrometry technique to measure free radicals in alf alfa seeds as a means to deter- mine whether the seeds have been irradiated or not is feasible, relatively fast and simple.