Objective　To investigate the morphological differences between  the body of  the type I lumbosacral transitional vertebrae(type I LSTV) with that of the fifth lumbar vertebrae for the clinical diagnoses of  the type I LSTV.　Methods　60 cases of the type I LSTV and 60 specimen of the fifth lumbar vertebrae were obtained, all of which had relative intact structure in birth. All specimen were measured by the uniform criterion for the following items: (1) the anteroposterior diameter and the transverse diameter of the vertebrae(both sides); (2) the anterior-vertical diameter, the posterior-vertical diameter and the anterio-posterior vertical diameter ratio of the vertebrae. The pedicle diameters of the type I LSTV All data were analyzed with the SPSS13.0 by the independent sample t-test.   Results  Compared with the fifth lumbar vertebrae, the superior antero-posterior diameters of the type I LSTV  are 1~2 mm longer in average than the normal L5; its inferior antero-posterior diameters, longer 1mm; its superior transverse diameters, 2～3 mm longer; its inferior transverse diameters, 2~3 mm longer; its anterior-vertical diameters, 0.7~1.2 mm higher; its posterior- vertical diameters, 0.8~1 mm higher. There is significant difference between the type I  LSTV and  the normal L5（P<0.01）; There are no significant differences between each group of the those in the anterio-posterior vertical diameter ratio of the vertebrae.    Conclusion    Most diameters of  the type I LSTV is larger than the normal L5 but they have the similar shapes, it is difficult to identify them only on the lumbar lateral X-ray, CT and MRI film because of  those similar  morphologic characters . The pedicle  screw of the lumbosacral transitional vertebrae type I is the same as that of the normal fifth lumbar vertebrae in clinical practice.