Free radicals were studied at room temperature using ESR spectroscopic method after wheat embryos were irradiated with argon ion beams with medium energy (20.7 MeV/u). The stability of the radicals was observed. Experimental results show that the radical concentration in seeds linely increases with moisture content of seeds: the radical concentration increases with irradiation dose and gradually reaches a saturation state. The relatioship of the radicals and damage to seeds was discussed. According to experimental data and radical diffusion theory the diffusion distance of the radicals in wheat embryos was estimated.