Abstract: A new dosimeter of 251 I in thyroid gland is described. The dosimeter consists of NaI(Tl) detector and intelligent data recorder. Single chip microcomputer is used for data handling. The activity of 125 I in thyroid glands of human being is measured directly, rapidly, and accurately. Furthermore, it can calculate and display the intake, comitted dose equivalent and committed effective dose equivalent. The meauring range of 251 I in thyroid glands is 10～2×10 6 Bq. The dosimeter has been operating continuously for a long time with high stability.