Spinal fixation using pedicle screws has recently been the focus of increased attention, but the adequate size of pedicle screw and maximum percentage fill as related to the pedicle diameter and are not well known. The objects of this study were to determine the ideal ratio among pedicle, drill and screw diameter, and to determine the maximum percentage fill of the screw without significant decrease of pull-out strength. The materials used for the experiments were 376 thoracic pedicles obtained from the 38 young pigs, and the diameters of pedicles ranged from 3.0 to 8.5mm. After 40% to 100% drilling as compared to pedicle diameter, screws were inserted carefully, and measurements were taken of the outer pedicle changes and pull-out strengths, and adequate drill and screw sizes as related to the diameters of given pedicles were determined. It was found that pull-out strength was the strongest after 60% drill, and the larger the drill diameter, the smaller the holding power, and the larger the screw diameter, the greater the holding power. Maximum pull-out strength was seen at 80-90% fill with 60% drill. After sequentially drilling each pedicle with increasingly larger drill bits, larger screws could be inserted with pedicle changes such as expansion, cutout, split fracture, and comminuted fracture. after larger drilling up to 100%, pedicle screws with diameters smaller than 115% of measured pedicle diameters could be safly inserted without fracture and significant decrease of pull-out strength. It is concluded that effective percentages of drill and screw diameters to the pedicle diameter are 60% and 80-90% respectively, and pedicle screw up to 115% of measured pedicle diameter can be safely inserted into pedicle without significant decrease of pull-out strength. It is thought that fresh pedicle has elasticity and larger screw can be inserted to the pedicle with strong holding after larger drilling.