Irradiated cells express DNA structural damages. such as DNA double-strand break. DNA-DNA crosslink and DNA-protein crosslink etc. These damages result to changes of DNA supercoil state, which trigger a series changes of DNA replication and expression. Many methods to test DNA damages have been established, roughly can be devided into two kinds according to the DNA materials used. The first use the naked DNA extracted from cells and free from DNA-binding materials existed in cell. The second one uses nucleoid for research, here the detergents and hypertonic salt buffers were used to remove nuclear envelope and a part of nuclear proteins, nuclear DNA remains appropriate tangled loop and binds to residual nucle-arskeleton. This DNA structure is beneficial to research damage effects on DNA structure, single cell gel electrophoresis belongs to the latter. It also named comet assay because its cell electrophoresis shape looks like a comet. It can test not only DNA stand break but also measure DNA structure changes resulted from stand bleak. According to oversea reports, with slight modification, single cell gel electrophoresis assay has been established Employing image analysis system. fast quntitative measurement of DNA structure changes of single cell irradiated as low as 0.1Gy can be given with a well correlated dose-respones relationship. After further study, the method might be developed as a kind of biodosimeter for application of monitoring enviromental low level irradiation.