Abstract: The electron spin resonance was used to trace the free radical concentration in the seed of Sesamum indicum treated with magnetic field once a day for 14 days. The results show that the free radical concentration in the Sesamum indicum seeds increases with the magnetic field treatment, and decreases obviously with the prolongation of storage period and loses more than 30% 14d later. The experiments with the optimum intensity of magnetic field (835G) to treat Sesamum indicum seeds for 12min, 24min, 36min, 48min and 60min demonstrate that the treatment time between 36 and 48min can get an ideal biological effects.