The frequency of single strand breaks (SSBs) occurring on both strands of the pBR322 plasmid DNA region flanked by a pair of primers used for polymerase chain reaction (PCR) amplifications was determined after irradiation with 137Cs y rays. We verified that real time PCR is suitable for the detection and quantitative analysis of SSBs caused by y ray irradiation. The utility of this approach was also supported by the comparison of the practical experimental data with the Monte Carlo simulation. The potential application of this PCR method for the detection of genomic DNA damage was also confirmed.