Introduction: Investigation of the blood flow in the cerebral arteries has important clinical applications. There is a dearth of research on fluid flow in the circle of Willis, analysis of shear stress on the arterial wall, and the effect of hyperelasticity of the arterial wall and fluidstructure interactions in different gravities. This study has practical implications in aerospace medicine. Materials & Methods: In this study, Computational fluid dynamics methods were used to study the blood flow in the cerebral arteries and the stresses on the arterial walls through alternations in gravity. The circle of Willis was introduced as a flexible tube with hyperelastic material properties. The solution of the flow was evaluated using the method of fluidstructure interactions in two gravitational accelerations of zero and 9. 8 m/s2. A total number of 248 computed tomography angiography images were used to design the geometry. The boundary conditions considering the multi-branching and autoregulation were assumed at the inlet and outlet of the arteries. Findings: Regarding the 9. 8 m/s2 gravity, the maximum stress was equal to 3. 9 Pascal. On the other hand, when gravity was neglected, the corresponding value was equal to 6. 5 Pascal. Considering the results of blood flow in 9. 8 m/s2 gravity, the blood pressure in the upper arteries and the circle of Willis was significantly reduced, compared to the blood pressure output from the heart. Discussion & Conclusions: The rate of production of biochemical materials due to a mechanical stimulation had a direct relationship with shear stress. Therefore, it is anticipated that those chemical reactions occurred more in the anterior and posterior communicating arteries. The results of this study can be used in physiological experiments on the astronauts, mechanobiological studies of the cerebral arteries in pathological conditions, and investigations of tissue growth and repair in regenerative medicine.