This paper deals with the determination of the seat sponge parameters by using the estimated nine degree-of-freedom biodynamic model. The suggested nine DOF model has multiple outputs that include the major axes for evaluating the ride quality in vehicles such as z-axis of the floor, hip, and x-axis of the back, in addition to the z-axis of the head for describing the whole-body vibration. It is intended to resemble the sitting posture with backrest support. Three experiments were executed to validate the proposed models. The first one was to measure the acceleration of the floor and hip in z-axis, the back in x-axis, and the head in z-axis under exciter. From this measurement, the transmissibilities of each subject were obtained. The second one was the measurement of the joint position by the device having pointer, and contact point between human body and seat by pressure sensor. The third one was the dropping test to measure the seat and back cushion. The biodynamic model parameters were obtained by matching the simulated to the experimental transmissibilities at the hip, back, and head.From this estimated biodynamic model, the optimal seat parameters are determined to minimize the overall ride value at the floor, the hip, and the back on three different road - highway, Korean national road, and unpaved road, with the constraint that the stiffness of sponge has a roughly linear relation with the damping, when making a sponge form by changing the mixing ratio, the index, and molded weight. The optimal seat sponge characteristics is found that the lower stiffness and damping transmits the lower vibration to human body.