The aim of this study is to determine whether the posterior cervical fusion methods with the plate-screw system applied to the lateral mass of cervical spine are radiologically safe to patients. The lateral cervical X-rays and CT scans were done on 40 normal adults without cervical problem. Based on Roy-Camille and Magerl's method, the theological trajectory of screw was shown on films and the parameters were measured. The study based on Roy-Camille's method showed less than one percent chance of injury on the facet joint and the mean depth of the screw to be 10.5±1.4mm. On the other hand, Magerl's method showed the mean depth of screw to be 11.9±1.5mm which is slightly larger than that of Roy-Camille's method and no chance of facet injury occurred. A reduced lateral angle of screw (19.6±3.5°) performed with the concept based on Magerl's method resulted a longer depth of screw (13.5±2.1mm). Both Roy-Camille and Magerl's methods seemed to be radiologically safe to normal persons. However, the authors recommend the reduced lateral angle (19.6±3.5°) of screw based on the Magerl's method more than an original Magerl's methods.