Abstract: The Willis Circles in 25 tree shrews were studied.Latex mixed with China ink was injected into the ascending aorta of all animls.Under the anatomical microscope,the main branches of the Circles were observed and measured.There is a complete Willis Gircle in most of the animals (88%).The anterior cerebral artery of each side sends off a median branch,and both branches form a common anterior cerebral artery.These two branches may be called anterior communicating arteres.In addition,right and left posterior communicating arteries communicate internal carotid arteries and posterior cerebral arteries which come from basilar artery.The calibre of anterior and posterior communicating arteries are sufficiently enough,therefore if one wants to make a whole cerebral ischemia of tree shrew one has to ligate all the four arteries,I.e.,the right and left common carotid and vertebral arteries.