In　order　to　get　three　dimensional　measurements　simply　an　plain　X-ray　photographs.The　multi-layers　of　bone　pieces　were　made　by　use　of　bone　saw,　and　piled　up　to　spiral　ladder,　which　was　used　as　a　calibrating　module　for　to　living　bone　in　clinics.　The　relationship　between　the　cortical　thickness　and　gray　level　of　the　X-ray　photographs　was　obtained.The　correlation　can　be　described　with　a　cubic　regression　curve,　and　it　can　be　simplified　to　a　linear　regression　equation　when　cortical　thickness　is　small　than　3.8　mm.It　can　be　realized　to　get　three　dimensional　measures　of　the　bone　on　plain　X-ray　photographs,　by　use　of　the　bony　spiral　ladder　described　here.