There exist residual stress in rat trachea. When a no-load trachea ring is cut radially, it opens into a sector, which is the zero-stress state of the trachea. Results in this paper show that for the no-load ring of rat trachea, the opening angle obtained for cartilaginous cut differs from that for muscular cut, i.e. the opening angle. for cartilaginous cut is significantly larger than that for muscular cut. And the opening angle obtained from muscular cut is basically identical along longitu- dinal location and that from cartilaginous cut tend to increase with the longitudinal distance x/L increasing. The results are helpful for studying adapting growth of the trachea in response to certain mechanical environment.