The purpose of this study was to demonstrate the feasibility of 4D Flow MRI to monitor the change of flow dynamics between before and after extracranial-intracranial (EC-IC) bypass surgery. We enrolled 14 patients who underwent EC-IC bypass. In all, 5 patients underwent radial artery graft (RAG) bypass and 9 patients underwent superficial temporal artery (STA) bypass. All patients underwent 4D Flow MRI preoperatively and 3 weeks after surgery. We measured blood flow volume (BFV) of bilateral internal carotid artery (ICA), basilar artery (BA), and EC-IC bypass artery using 4D flow MRI. Post/pre-operative BFV ratio of contralateral ICA was statistically higher in patients with the RAG bypass group than in those with the STA bypass group (1.28±0.26 in RAG bypass group vs 0.94±0.14 in STA bypass group, p<0.01). 4D Flow MRI could assess comprehensive flow dynamics of EC-IC bypass surgery. It is feasible to clarify the change of contribution from each artery.