Abstract： Objective To investigate anatomic variation of the pilot's circle of Willis (CoW).Methods Case files and magnetic resonance angiography (MRA) images,which were collected from 104 hospitalized pilots in the period of 2000 -2011,were retrospectively analyzed to investigated the integrity and anatomic variations of CoW upon the screening out of cardiac-cerebric angionosis,hypertension,diabetes,neural afunction and significant abnormality by head magnetic resonance exam.The integrity and anatomic variations of CoW were statistically analyzed. Results Fifty-one suitable cases,including 29 fighter pilots,19 bomber or transformer pilot s and 3 helicopter pilots(aged from 22-56 yrs and had flying hours from 100-8000 h),were selected.Among which,41 cases were hospitalized due to health and other 10 were for the physical exam of transformation.Only 37.3％cases (19/51) showed integral CoW while anterior communicating artery (ACoA) absence,dysplasia at A1 segment of anterior cerebral artery (ACA), unilateral A1 segment absence, posterior communicating artery (PCoA) absence,dysplasia at P1 segment of posterior cerebral artery (PCA)and unilateral or bilateral P1 segment absence was respectively 23.5％ (12/51),7.8％ (4/51),2.0％(1/51),51.0％ (26/51),7.8％ (4/51) and 3.9％ (2/51). Conclusions Anatomic variation exists in counted pilots and more than half of them show CoW imperfection.Whether applying such variation in aviation medicine assessment is still to be determined.