The radioprotective effect exerted by hydroxy cinnamic acid derivatives on the radiation damage of DNA, such as double-strand break (DSB), intermolecular crosslinking (CL), has been studied by gel electrophoresis. The molecular length distribution of DNA can be measured by this method, and by comparing average molecular length of DNA before and after irradiation, the frequency of DSB and CL can be determined quantitatively. The radioprotective effect of caffeic acid and sinapine on calf thymus DNA in binary system at room temperature have been determined. Both caffeic acid and sinapine reveal apparent protective effects and the former is stronger than the latter. The mechanism of radioprotection at room temperature has been discussed. Based on this finding, combined with the previous ESR study and an analysis of the molecular structure about these two protectors, it is suggested that the target is protected as a result of hydrogen atom transfering from protector to target.