Effect of lipid dietary status and cumulative doses of whole body gamma irradiation up to 15 Gy, on histological pattern of the dorsal aorta of male albino rats was studied. Rats fed fat-free diet showed enlarged spaces filled with amorphous fibrous material lying in-between smooth muscle cells, whereas animals fed fat-rich diet showed none of these changes. Rats fed normal fat diet and subjected to the fractionated dose levels showed structural changes in all the 3 layers of the dorsal aorta. Thickening of both tunica media and adventitia, degenerative endothelia cells and granulomatous inflammation of the adventitia were observed. Irradiated rats fed fat free diet showed more intensive structural changes. These included degeneration of endothelial lining, disorganization of smooth muscle cells and vacuolation in tunica media and degenerated muscle cells. Condensed elastic and collagen fibers could be observed in-between smooth muscles and tunica aventitia looked degenerated with less numbers of fibroblasts. Irradiated rats fed rich-fat diet showed rapid structural changes. These included degeneration of endothelium of tunica intima, reduced numbers of smooth muscle cells, hyaline degeneration of tunica adventita with signs of necrosis and fragmentation, decreased thickness of the dorsal aorta cells and ill-defined separation between the tunica media and adventitia.