Abstract: Cell responses to ionizing radiation are determined by extent and repair capability of DNA damage.Capability of DNA damage repair and character of damage vary as a function of cell cycle. Thus cell kinetics alteration is the principal factor that influences the effect of radiotherapy. With advancement in radiobiology, the respective molecular mechanisms underlying cell kinetics alteration and DNA damage repair is being revealed gradually and takes on many obvious similarities. We review the recent achievements relevant to radiotherapy in molecular biology to show potential relationships among them.