Abstract: The zero stress state of an artery is brought by cutting a ring of the artery radically and, then, the ring segment springs open into a sector. The zero stress state is a foundation of strain analysis of an artery. There are two methods to measure the dimensions of a blood vessel in the zero stress state. First, it is a way to measure immediately an opening angle. Second, it is a method of assuming that a cut-open, stress-free specimen forms a circular sector and there are two edge angles. A new method to measure the dimensions of an arterial wall in the zero stress state is proposed. The new method is to measure immediately the curve length of an arterial wall in zero stress state by computer image process (CIP). The curve lengths are used to define a standard circle for the strain analysis. These three methods on descending thoracic aortas, common carotid arteries, and femoral arteries of white rabbits are compared with each other. Conclusions are: (1) the new method, i.e, the strain analysis method from the curve length, is the best one among the three methods of strain analysis. (2) The lengths of a ring segment (the length between D/2 and 2D) do not affect the dimensions of the zero stress state.