Willis' Circle of 162 Chinese brains was studied and the diameters of thecomponent arteries of this circle were measured in 51 brains for the purpose ofobtaining a pattern of this circle in Chinese and, in certain types, the possibleresult after the ligation of one side of the internal carotid artery. For compari-son, 12 dogs' brains were also studied. It was found that this circle may be divided into a fore part and a hind part,each of which may be again divided into a trunk trpe, a diffuse type and anintermediate type. In the fore part, the trunk type consists of 67.9%, the dif-fuse type 22.21%; the intermediate type 9.88% In the hind part, the followingtypes were observed: first subtype of the trunk type 83.3%; second subtype ofthe trunk type 6.17%; diffuse type 6.79%; intermediate type 1.735 and open type2.47%. The posterior communicating artery was found to be poorly developedor absent in 51.26%. In the dogs' brains there are two circles, one in front ofthe other. The front one is polygonal and the other one is rhomboid in shape,with a constant posterior communicating artery so that the circulation is morestablized as compared with that of the human brains.