Purpose: Computational Fluid Dynamics has been introduced to analyze the hemodynamics of intracranialaneurysm. This study aims to produce a critical assessment to evaluate the risk of bifurcation type MCA aneurysms. Methods: A series of in-vitro MCA models were designed with aspect ratio 0.8- 3 and the aneurysm's size was selected 3-10 mm. Energy loss (EL) was calculated. The EL was indicated that the energy balance when flow pass through the aneurysm, we hypothesized it as a critical state. Once, EL was developed to be higher than the critical EL, the surplus of power must be transferred to harm lumen surface, here a range of hemodynamic-related factor might be identified to predict rupture risk. To confirm our hypothesis, two ruptured MCA aneurysms which had suffered from two stage growth and finally ruptured, and five unruptured MCA aneurysms were analyzed. Results: The EL of in-vitro models were calculated at each aspect ratio. The EL of ruptured aneurysm increased to 2 times beyond the EL range of the in-vitro models. Contrarily, after its rupture, the EL dropped dramatically to values lower than that of the in-vitro group. The result indicated that high flow energy was concentrated inside the aneurysm before rupture, and that hemodynamic power was released after SAH. The energy loss calculated from other five unruptured aneurysms appeared lower than that of the in-vitro group; below the critical EL. Conclusion: When the EL is observedto increase rapidly at the follow-up growth aneurysm, it may be an indication of the higher risk of aneurysm rupture. The EL of the in-vitro group may be used as a critical assessment tool to evaluate the rupture risk of MCA aneurysms.