STUDY DESIGN: Lateral mass was measured using MRI for lateral mass screw fixation. OBJECTIVES: To measure the lateral mass of cervical spine using MRI for lateral mass screw fixation and find out the ideal entry point and insertion angle and length of lateral mass screws. SUMMARY OF LITERATURE REVIEW: Two methods of screw placement are in common use. The original technique, described by Ray-mond Roy-Camille, places the screw in a more or less straight sagittal direction and angling the screw laterally 10 to 20 degrees. Margerl technique involves placing the screw parallel to the facet joint and angling the screw laterally 25 to 30 degrees. MATERIALS AND METHODS: Axial MR images of the cervical spine parallel to the facet joints were obtained from C3 to C6 of 10 patients. The mean age of the patients were 48.0 years. The patients consisted of 6 male and 4 female patients. Ideal entry points, insertion angle and length of the lateral mass for lateral mass screw fixation were measured on the axial MR images using PACS digital measuring instument . RESULTS: Ideal entry point of a lateral mass screw was center of lateral mass in sagittal plane, 16mm lateral to the midline of the cervical spine, ideal direction of the lateral mass screw was parallel to the facet joint and angling the screw laterally 25.3 degrees, and ideal length of lateral mass screw was 17.9mm. CONCLUSIONS: Based on the results of the study, there were some differences of measurements depending on the patients and the level of the cervical spine. Therefore, a preoperative measurement of lateral mass was recommended in each patient and each level of the cervical spine.