The thymus of twenty Awassi sheep and a street dog is used for this study. The age of animals was two months and one year in both species, and each age had five animals. The gland consisted of cervical and thoracic lobes in sheep but in dogs have thoracic lobe only. The maximum value of length and weight was in puppies were (7.22±0.295cm and 17.760±0.3286g) respectively while the value of the size was maximum in lambs (15.26±0.182 ml) at P ≤0.05 value. As the age of animal was developed the thymus was regressed and become very small. The histological and histochemical, characterized by a thin connective tissue capsule with collagen and a few reticular fibers and the highest value of capsular thickness was in adult dogs (45.9±21.70 µm). The cortical thickness in the puppies had the highest value (124.7±49.01 µm), while the medulla thickness was maximum in the lamb (377.4±510.74 µm). Thymus of adult dogs lost the demarcation between the cortex and medulla and appeared as the lymphoid follicles surrounded by epithelial reticular cells and connective tissue with thinner thickness in cortex and medulla (15.1±8.04 µm and 36.6±8.06 µm) respectively in compared with adult sheep. T cells give positive reaction to the CD3 antibody in both species; the highest density and strong intensity was in lambs. The macrophage gives positive staining to the CD68 antibody and appeared more distribution and strong intensity in the thymus of adult dog while the sheep give negative reaction except in medulla in adult sheep.