Abstract: The effective half-life Teff of 131I in patients with differentiated thyroid cancer was determined by monitoring the radiation dose rate around the patients at different time after radioiodine-131 therapy. The average Teff of 131I for the 21 patients post thyroid surgery was estimated to be (14.6±6.5) h. The value is similar to those reported by other authors. It indicates that no difference of 131I Teff exists among different races. The results of statistical analysis show that negative correction exists between the Teff and the age of patient, the number of times of radioiodine therapy. On the other hand, no significant difference is observed between the Teff and the gender, the administered activity.