The objective of this research is to analyze vibration mechanisms for developing a seat of high speed railway vehicles which provides good ride comfort to passengers in the sitting state. Therefore, this paper describes the modeling of dynamics model of the seat and the passenger in the sitting state. First, seat excitation experiments are carried out to acquire high frequency vibration characteristic of passenger and seat. Next, a passenger model is built based on the characteristics, and the governing equations of the model are derived. Next, the parameters of the passenger model are identified from the experimental results as well as literature survey. Finally, simulation results reveal that the formulated equations of motion can express the actual passenger motion.