The purpose of this study was to clarify the change of hemodynamics after high-flow extracranial-intracranial (EC-IC) bypass surgery for ICA aneurysm by using time-resolved 3D-phase contrast (4D Flow) MRI. We enrolled 11 patients who underwent high-flow EC-IC bypass surgery. They underwent 4D Flow MRI before and after the surgery. We evaluated the blood flow direction of the circle of Willis. We measured blood flow volume (BFV) of bilateral ICAs, BA, and bypass artery. Seven of 11 patients exhibited collateral retrograde flow in the circle of Willis after surgery. The BFV of contralateral ICA and BA, and total brain BFV statistically increased after surgery. While, there was no evidence of post-operative hyperperfusion in any cases. 4D Flow MRI could quantify the change of hemodynamics after the high-flow bypass surgery.