Thymus of 30 native male and female Iranian sheep, 5 months to 8 years old, were studied. Thymus in the sheep is located in cervical region on either sides of the trachea and in thoracic cavity in the left cranial mediastinum. The thymus were anatomically studied and the dimensions of different parts were measured and weighed separately. After fixing in formalin, 6 micron sections were prepared and stained with H&E and studied under the light microscope. The regression of thymus was correlated and associated with the increasing age of the animal. It was observed, that in addition to morphological changes, the ratio of the thymus to body weight, decreased, as the age of the animal increased. Histologically, the thymus showed, a progressive fatly changes from capsule to cortex. There was also a marked increase in the number of apoptotic lymphocytes and the size and number of hassal bodies.