Recently, there are many considerations and studies on biological effects of radiations in radiation workera, as well as in accidentally or therapeutically irradiated persons. The most practical and reliable method of dosimetry for radiation accidents is the scoring of gross chromosomal aberrations in human lymphocytes (Ydr) as a biological dosimetry. By the way, although usual doses of 131I administered therapeutically for thyroid cancer are ranging from 100 mCi to 200mCi, there are differences of absarbed doses and Ydr, ranging from 0.004 to 0.04, on equally administered 131I due to variations in metabolic characteristics, stage of tumors and physical status of subjects. In this study, We exert to obtain the dose- response relationships of 131I, as a good guide to evaluating acute effects of accidental irradiations and radiation induced leukemia or solid tumor, by in vitro induction of chromosomal aberrations. we studied the relationship between radiation dose