A new and reproducible saccular aneurysm model has been developed at the bifurcation of the common carotid artery in rats. The details of the experimental methods and results are described. It is strongly suggested that the internal elastic lamina is a critical layer in saccular aneurysm formation, because an experimental saccular aneurysm can be produced immediately by transluminally damaging the inside of the arterial wall at the bifurcation of the common carotid artery. This saccular aneurysm model has several advantages: (i) it can be induced quickly and the success rate approaches 100% in rats; (ii) this technique can produce satisfactory experimental saccular aneurysms for other aneurysm studies, and in the future it will also be possible to use this technique to produce experimental saccular aneurysms in cerebral arteries of large animals.