Morphological characteristics associated with cerebral aneurysm formation can be used to assess aneurysm rupture. This study investigated hemodynamic effects resulting from change in the parent artery diameter of bifurcation type aneurysm. Computational fluid dynamic (CFD) analysis was performed on middle cerebral artery (MCA) models with various parent artery diameters. Calculations were performed with steady flow rate (125±12.5 ml/min) at the parent artery inlet. Energy loss (EL) was calculated from pressure and kinetic energy obtained from flow velocity. The results indicate that the high wall shear stress (WSS) and EL occurs in model with the smallest parent vessel compared to the other models for all three inflows. Results also showed that 10% variation of inflow results in average of 23±2.9% changes in WSS and 25.5±0.5% changes in energy loss. These results demonstrated that for CFD analysis of MCA bifurcation type aneurysm, upstream parent vessel and inflow evaluation for individual patient is essential.