Comparative studies on frequencies of micronucleus (MN) and cells with MN from human blood lymphocytes in vitro which exposed to low dose radiation were carried out using conventional methods for culture time 50 h and 72 b and cytokinesis-block micronucleus method (CBMN). The experiment results showed that the frequencies of MN and cells with MN increased with dose. Dose-response relationship for conventional method of culture time 50 h and CBMN method were linear model, while culture time 72 h was quadratic ones; CBMN method is more sensitive and precise than conventional 2 methods; The frequencies of MN and cells with MN could be unequivocally detected if the blood was exposed to 0.1 Gy of ,137,Cs γ-rays in vitro.