The hypocotyls and cotyledons of ,Medicago sativa, were irradiated with original energy 100 MeV/u heavy ion beam ,12,C,6+,. The effects of irradiation on the growth state and the callus induction rate, the callus relative growth rate, the somatic embryogenesis rate and plantlet regeneration were studied. The results show that heavy ion beam irradiation can inhibit the callus induction of the hypocotyls and cotyledons, and the frequency of callus declines gradually with the increment of irradiation doses. In the course of subculture, their callus relative growth rates are higher than that of the controls, and the hurt caused by irradiation has been restored. The heavy ion beam irradiation has an influence on the somatic embryogenesis rate. The somatic embryo induced by the hypocotyls irradiated up to 30 Gy formed earlier and the number is larger while irradiated at 50 Gy there is no plant obtained. The results of RAPD (Random amplified polymorphic DNA) analysis on the control and the plants induced by the hypocotyls irradiated within 10~40 Gy suggest that eleven among twenty random primers have been amplified with different DNA polymorphism bands. Consequently, heavy ion beam irradiation could cause mutation of alfalfa genome DNA.