Micronucleus (MN) assay as a routine examination for genotoxicity has been widely used. The testing specimens were taken from bone marrow and extended from blood and tissues. In addition to testing genotoxicity of drugs, it is also applied in disease diagnosis for genetic mutation, evaluation of curative effectiveness and disease prevention. Moreover, MN assay is also an important safety indicator for drugs and health foods registration. This review will discuss the staining method of MN test and its application in the field of diseases and virology.