Objective　To clarify the technical parameters of C1 lateral mass screw in anterior Jefferson-fracture reduction plate. 　Methods　Anatomic structure of C1 lateral mass using 1-mm CT scans were measured with MIMICS software in 80 C1 lateral mass of 40 cases. 2 entry points were selected for superior and inferior screw placement and screws (2.5mm in diameter) , and simulation procedure was performed at these entry points. Length, angle of screw and relation with adjacent anatomical structures were recorded.　Results　Distance from medial wall of vertebral artery hole to midline of atlas was 23.2 mm. Distance between medial wall of posterior arch junction with lateral mass and midline was 13.2 mm, and from entry point of superior screw to anterior edge of superior articular and midline were 6.2mm, 20.0mm respectively. The length of superior screw was 21.5 mm and the range of sagittal angulation was upward 1.5° to downward 11.6°. Distance from entry point of inferior screw to anterior edge of inferior articular was 8.9mm. Length of inferior screw was 15.2mm. The largest downward angulation for inferior screw was 20.7°. Distance from medial wall of vertebral hole to inferior screw was 1.9mm. The area of lateral mass (17.6~23.2) mm away from midline of atlas is relative safe for screw placement.　 Conclusions　It is feasible to place C1 lateral mass screws (superior and inferior screw) in anterior Jefferson-fracture reduction plate. Making final decision of entry points and screw direction must depend on preoperative 3-dimensional CT evaluation. According to our research, the angulation of superior screw below turn to downward, and the angulation of inferior screw turn to inside were recommended.