Medicai Internal Radiation Dose(MIRD) schema was developed for calculating the absorbed dose from the administered radiopharmaceuticals. With the biological distribution data and the physical properties of the radionuclide we can estimate the absorbed dose by the MIRD schema. For the thyroid cancer patients received 131I therapy, the absorbed dose to the bone marrow is the limiting factor to the administered dose, and the duration of admission is determined by the retained activity in the whole body. To monitor the whole body radioactivity, we used Eberline Smart 200 system using ionization chamber as a detector. With the time activity curve of the whole body, total body residence time was obtained. From the ICRP publication 53, the residence times of the source organs, such as kidney, urinary bladder content and stomach, were, used to calculate the absorbed doses of the target organs, such as stomach, red marrow, b1adder wall and remaineder total body. In 8 thyroid cancer patients with 175 mci of 131I adminstered orally, the mean absorbed dose in the blader wall was 375.1 in the stomach 285.1, red marrow 25.4 and total body 22.4 rad respectively. For the monitoring of the large administered activity, this method seemed to be quite useful.