Vertebral pedicle screws have been widely used for secure posterior spinal fixation. When postoperative CT scan films were made, the blurring of pedicle screws were observed and we could not figure nut the exact diameter of screw and canal encroachment. There is no information in the difference hetween actual diameter and measured diameter of pedcile screws in CT yet. In this study, we try to find out partial voiume averaging artifact of pedicle screws made of stainless steel and titanium. Partial volume averaging artifact occurs when the shape of an object changes within the thickness of the CT slice or when a relatively small object is only partially included within the slice. Four pig cadaveric spinal column including 6 vertebrae each were prepared and CT scans were performed after insertion of C-D screws(stainless steel) to the left and TSRH screws(titanium) to the right pedicle. Another CT scans were performed after insertion of C-D screws to the right and TSRH screws to the left pedicle. The third CT scans were made after removal of all the pedicle screws. Actual C-D screw diameter was 6mm and CT scanned diameter was 8. I 6±0.66mm. Actual TSRH diameter was 6.5 mm and CT scanned diameter was 6.59±0.34mm. In conclusion, stainless steel has more partial volume averaging artifact than titanium. Safety margin of pedicle screw made of stainless steel is about 2mm and that of titanium is about 1 mm.