Distribution of 99mTc-MIBI was studied in frozen section of rat thyroid tissue using autoradiography and γ counter detector at different times after cauda vein injection. The results demonstrated that the ability of thyroid to uptake 99mTc-MIBI was strongest at 3 minute after injection and the speed of excretion was slow in the period from 15 to 60 minutes and thus formed the retention period with the highest concentration. 99mTc-MIBI was rapidly eliminated after 60 minutes and autoradiographic film exhibited that large amount of 99mTc-MIBI distributed in follicular epithelial cells especilly in basal part.