Blood flow of lateral aneurysm modes with a straight/slight curved parent vessel was numerically simulated. The computations were carried out under incompressible, viscous, Newtonian flow, non-elasticity of the wall conditions by using an artificial compressibility algorithm to solve three-dimensional N-S equations. Flow pressure, wall shear stress, inflow volume and velocity/pressure field were calculated in the steady and pulsatile conditions. The maximum /minimum Reynolds number was set at about 475/152 based on diameter of parent vessel. The Womersley number was set to 2. 5. These values are a representative of the middle cerebral arteries in vivo data. The significant results show that lateral saccular aneurysms subjected to wall shear stress concentration at proximal and the distal ends of cavity neck and more concentration for a straight parent vessel than slight curved vessel. It is observed that higher inflow rate from parent into cavity for slight curved than straight parent vessel in the most of time during a flow cycle. Two maximum pressure appear at proximal and aneurysm dome respectively, and rapid changes of blood flow direction and inflow quantity during a flow cycle result in rapid changes in inflow volume, wall shear stress and pressure at the proximal and distal wall of the cavity. The rapid changes would make continuous damage to the intima at the cavity neck and could lead to subsequent aneurysm growth or regrowth.