Water Jet flow was projected at normal human aortic walls and human chronic obstructive iliac arteries in the air or in the ordinary saline solution. Ordinary saline solution was used for the jet, which was projected at a pressure of 10kg·f/cm² through a nozzle 0.10mm in diameter. When the Water Jet was projected at the normal aortic intima, damage to the wall was more severe with duration of fluid projection, and projection for ten sec ruprured the elastic fiber of the media. But when the fluid jet projected ordinary saline, damage to the aortic wall was slight and projection for 30sec only dissected the surface of the intima. Water Jet projection in air showed slight effect on thrombi of the chronic obstructive iliac arteries, and projection for 60sec only made small irregular holes in the thrombi. When the Water Jet was projected in ordinary saline solution, however, destructive effects on thrombi were stronger and the projection for 40sec could remove almost all the thrombi for a distance of 2cm, while damage to the initima was very slight. This study demonstrated that fluid jet projection using ordinary saline solution could remove thrombi in chronically obstructive artery safely and effectively and suggested the possibility of the Water Jet angioplasty.