To observe the morphology and anatomic variations of the circle of Willis and its clinical significance. To measure and establish the normal reference parameters of average diameters of its component vessels in the healthy Chinese by using 3D-TOF MR angiograms and to determine whether age-or sex-related differences exists in the circle' anatomy. MRA was performed in 153 healthy subjects. According to its integrity, Willis'circle was divided into four types: (1) Type I, showing an intact circle. (2) Type II, having a complete anterior circulation but an incomplete posterior circulation. (3) Type III, having an incomplete anterior circulation but a complete posterior circulation. (4) Type IV, having an incomplete anterior and posterior circulation. Based on the development of the components of the circle, each type was subdivided into four subtypes: typical, variable, hypoplastic and mixed. The statistically significant differences were not found in each type or subtypes between the different age and sex groups. Result of 153 cases, Type I, II, III and IV was found in 53 (34.64%), 73 (47.71%), 8 (5.23%), and 19 (12.42%) respectively. The display rate of the anterior and the posterior communicating arteries was 87.58%o (134/153) and 65.34% (100/153) respectively. Display rate of fetaltype posterior cerebral artery(PCA) was 20.92%. 3D-TOF MR angiography has important clinical significance for evaluating morphology and anatomic variations of circle of Willis. The normal values of diameter of cerebral arteries on MR angiograms may play a reference role in diagnosing cerebral vascular diseases.