Abstract: The aim of study is to determine radio isotope absorption concentration by algae in drinking water. The drinking water with algae was prepared in the stream of Mahan City of Kerman Province, Iran. The case sub samples of drinking water were polluted by 0.5 mL of 125I among other radio nuclide 125, because of the low 125I activities in drinking water. Gamma counting was used as the determination method. Radio iodine in algae were radiated with gamma radiation and counted using gamma counting techniques. The source for the gamma emission measurement was prepared from a radioactivity aqueous solution. The activity concentration was calculated via gamma counting machine. Using evolution of 125I in the algae, absorption quantity of radionuclide was determined by 24%. The 125I activity concentrations were determined by the present study varied significantly, ranging from 135 to 210 CPM (count per minute. The high concentrations indicate an obvious lapse of environmental safety precautions. If the concentrations of Iodine-125 were so immensely high in these organisms, one shudders to think of those organisms where the radioactivity concentration of drinking water could not be so easily traced. Its conclusion can be realized that radio Iodine-125 would be absorbed by all parts of algae in the drinking water.