Abstract: The biological effects of ionizing irradiation is related with dose and dose-rate.Medium to large dose irradiation can induce damaging effect to biosystem.but low dose radiation may induce adaptive response,hormesis,hyper.sensitiVeness and bystander effeeL Especially experimental data suggest that the antitumor effects of immune enhancement,intrinsic hypersensitivity,activation antioxidase enzymatic system and induction of apoptosis is produced by low-dose total body irradiation.Furthermore it call decrease relative cancer risk in exposed populations.Therefore.the antitumor effect by low-dose irradiation is being widely investigated and intensively researched.