The dependence of ,60,Co γ-ray induced irregularities (chromosome aberrations and micronuclei) in human peripheral blood lymphocytes in vitro on radiation doses has been studied. All blood samples were irradiated with lower doses (0-60 rads) of ,60,Co γ-ray (dose rate 70.05 rad/min) at 37±0.5℃, cultured for 50 hrs by microtechnique; and observed in the same smears.,The obtained data concerning chromosome aberrations and micronuclei were fitted to the model Y=a+bD by least square regression analysis using Poisson variance and weights. And, a=a,0,=0, the regression was constrained to pass through the origin. We found a close relation (r=1.000)between the index of chromosome aberration and that of micronucleus obtained in different radiation doses existed. These experimental results indicated that the biological indexes of chromosome aberration and micronucleus, in the diagnosis of radiation dose, were iso-effective for "Biological Dosimeter", they were equally significant and valuable. Considering both the chromosome aberration and micronucleus, the level of lower dose diagnosis, was ranged from 60 to 5 rad.