With a model system of pBR322 plasmid DNA solution in vitro, the dose effects of radiationinduced single- and double-strand breaks (SSB and DSB) were measured and DSB was distinguished into a- and b-types. Under the condition of low scavenging capacity existing in the irradiated DNA solution, SSB and aDSB were mainly induced by hydroxyl radicals (•OH). Moreover, a certain relationship was obtained between the SSB and aDSB yields and the DNA concentration. It was found that when the DNA solution was irradiated in the presence of 2.5 mmol dm-3 mannitol, the reciprocals of G (SSB) and G (aDSB), respectively, were linearly related to the reciprocal of the DNA concentration, i.e. the competition reactions of DNA and mannitol for •OH radicals can be described by second-order kinetics. The rate coefficients and the efficiencies of the •OH radical inducing SSB were deduced. Also, the reaction rate coefficients and the efficiencies for the induction of aDSB from SSB by the •OH radical transfer mechanism, were first derived from the competition kinetics.