To estimate haemodynamic modification of Internal Carotid Artery (ICA) after bypass surgery using computational fluid dynamic (CFD) technology and thereby aid in our understanding of the influence of hemodynamic parameters on the outcomes of bypass operations. 18 patients who underwent superficial temporal artery to middle cerebral artery bypass and encephaloduromyosynangiosis (EDMS) surgery were included. Reconstructed three-dimensional vessel geometries from MRA were segmented to create computational domains for CFD simulations. All cases were classified as three groups according to the proportion of the MCA area of distribution supplied by revascularization: A, more than two thirds; B, between two-thirds and one-third; and C, less than one-third of the MCA distribution. Pre-operative and follow-up haemodynamic parameters, especially volume flow rate and pressure drop index (PDI) in ICA were compared. For all cases, PDI and volume flow rate in the surgical-side ICA decreased significantly at follow-up (P<0.05). For the cases of group A, volume flow rate in surgical-side ICA decreased by average 24.2%, whilst for the cases of group B and C, flow rate reduced by 10.5% and 3.7%, respectively. An average PDI for cases in group A was -1.67. mmHg, conversely average PDI values of group B and C were -0.53 and 0.82. mmHg, respectively. The remodelling of ICA after bypass was associated with reduction in the volume flow rate and pressure drop. Good correlation with angiographic grading suggested that CFD might play a critical role as a quantitative haemodynamic technique for predicting treatment outcome during the follow-up of MMD patients.