We investigated flow patterns in an intracranial side wall aneurysm model under pulsatile flow conditions using Particle Image Velocimetry (PIV). The hemodynamic parameters were representative for those in human vertebral arteries. Analysis of flow patterns show the formation of a vortex and shifting of its center inside the aneurysm during the pulse cycle. Higher than normal values of shear stress were recorded on the proximal and distal neck of the aneurysm during the systolic phase of the cycle. Maximum values were attained during peak systole. However, the stresses on the distal neck of the aneurysm were always higher than the those proximally.