As　is　well　known,　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.　In　this　paper,　we　proposed　a　new　method　to　measure　the　dimensions　of　an　arterial　wall　in　the　zero　stress　state.　The　new　method　is　to　measure　immediately　the　curve　length　of　an　arterial　wall　in　zero　stress　state　by　computer　image　process　(CIP).　We　used　the　curve　lengths　to　define　a　standard　circle　for　the　strain　analysis.　We　compared　these　three　methods　on　descending　thoracic　aortas,　common　carotid　arteries,　and　femoral　arteries　of　white　rabbits.　Conclusions　are:　(1)　the　new　method,　that　is　the　strain　analysis　method　from　the　curve　length,　is　the　best　method　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.