Dry dormant onion seeds were irradiated by 0.2MeV neutrons from an accelerator, or ,60,Co γ-rays, for a better understanding of relative biological effectiveness (RBE) of mono-energy neutrons, and for observing different frequencies of micronuclei in root-tip cells of the onion seedlings induced by unit dose of the neutrons and γ-rays. The frequencies of micronuclei induced by unit dose of the 0.2MeV neutrons and the ,60,Co γ-rays are 5.36±0.30 and 0.031±0.002 (%Gy,-1,), respectively. The RBE value for the 0.2MeV neutrons is about 173±15, using the ,60,Co γ-rays as reference radiation. The RBE value of the 0.2MeV neutrons to induce micronuclei in the root-tip cells of ,Allium cepa, onion seedlings irradiated as dry dormant seeds is as high as 173±15. Our findings suggested that the repair efficiency of DNA damage induced by mono-energetic neutrons might be different from those by γ-rays.