The non-physiological motions of human cervical vertebrae were analyzed in volunteer tests for rear-end impacts and were considered to be an important parameter for neck injuries. The objectives of this study are to improve the Marko de Jager neck model using volunteer test data and to analyze the influence of horizontal and vertical accelerations on cervical vertebral motion.In the beginning of this study, a neck model was positioned based on X-ray cineradiography of a volunteer. Motions of each vertebra were compared with those of volunteer test data for low speed rear-end impacts (4, 6, 8km/h). In these comparisons, the differences of vertebrae motions between the neck model and the volunteer tests were found. To improve the validity of the neck model, the connection properties and the bending properties of the upper and lower vertebrae of the model were modified to increase rigidity.Using the modified neck model, simulation analysis was performed by changing horizontal and vertical accelerations to analyze the influence of seat property on vertebrae motion. The forces caused by contact with each adjacent facet of the vertebrae, vertebra angles and vertebra rotation center relative to adjacent vertebra were calculated to evaluate the severity for the vertebrae and to analyze the motions of the vertebrae just before facet contact. It was found that the facet force and the height of rotation center were influenced not only by horizontal acceleration but also by vertical acceleration.