Micronucleus is an abnormal structure in eukaryotic cells. It is the result of various physical and chemical factors,such as radiation and chemical drugs acting on dividing cells. It is generally believed that micronuclei are originatedin backward chromosomes and fragments that have lost centromere and are formed at the end of mitosis. However,other experiments suggest that interphase cells can also form micronuclei. In this study, human peripheral bloodlymphocytes were cultured in vitro and cyclophosphamide 20 ug/ml was injected at specifi c times of the cell cycle toobserve the relationship between micronuclei formation and cell cycle. The experimental results confi rmed that thecell cycle micronucleus formation.