To evaluate the influence of the DNA concentra-tion in the aqueous solution on DNA radiation damage, the plasmid DNA in the presence or absence of Mannitol (scavenger of free radical OH-) was irradiated by 7Li ions and γ rays at various DNA concentrations. Gel electrophore-sis analysis revealed that the DNA damage of single and double strand breaks induced by irradiation was dependent on DNA concentra-tion and became more severe at lower DNA concentration in the radiation experiment when all others parameters are the same. In the con-dition of γ-ray irradiation, most of double strand breaks (DSB) damage was neutralized and less associated with DNA concentration in the presence of mannitol. However, under 7Li irradiation, the DSB damage could not be cleared by mannitol but was gradually aggravated with de- creasing DNA concentrations. Our study sheds light on the underlying mechanisms in the DNA radiation damage process. And there are poten-tial significances for the human space flight, cancer therapy by heavy ions as well as the ra-diation security assessment.