The human aortic arch has significant torsion in addition to its large curvature. It was previously pointed out from some clinical observations that the torsion may be related to the development of the thoracic aneurysms. It is also known that the torsion affects the blood flow, which is suspected to be associated with various vascular diseases. In this study, a realistic three-dimensional computational fluid dynamics (CFD) model of a human aortic arch was constructed based on a set of MR images and the blood flow in the arch was simulated. The results showed that the torsion of the aortic arch centerline led to a complex localized distribution of the wall shear stress at the top of the arch. MRI based modeling was shown to be a powerful means to investigate computational fluid mechanical analysis of the blood flow.