This paper is the proposal about providing an optimal design specification by utilizing the analysis simulation in order to improve the dynamic comfort of seat. First, it is analyzed transmissibility of the model having excellent long-distance comfort in condition of the same vibration condition, and optimized specifications is proposed as DFSS approach to reduce the vibration transmissibility at high frequency. Final optimized SEAT index showed the effect of decreasing 6.5% compared to the current specification. In addition, the improved seat sample was confirmed that the performance was improved by SEAT index to figure measurement and subject evaluation.