國道客運為創新經營服務型態、提昇服務品質，相繼採用雙排式總統座椅提供運輸服務。然較少座位數之營運方式，是否一定對乘客有利，既有服務品質能否正面提升，對社會資源是否能夠有效使用。本研究首要目的即從提昇高速公路客運服務品質、資源有效運用的觀點探討此一最適座位數課題。構建之模式係以管制者觀點尋求社會總成本最小之班車座位數，包括基本模式與考慮擁擠成本之最佳模式兩部份，冀能將客運採少座位數方式載客，路線班次數增加，造成高速公路使用者旅行時間增長之擁擠成本，納入社會總成本之考量。模式推導結果顯示，在車輛供給無虞、乘客到達可立即被服務、時段乘客為均一到達等前提下，最適座位數與等車時間價值、等車時間對班距比值及合理乘載率的平方根成反比；與最大需求量、各時段需求平均、迴車時間、滿載係數、單位班次行駛成本、單位車輛成本、營運時間、路線單位班車擁擠成本的平方根成正比。本研究並以國道客運台北-台中線之營運資料測試，最適座位數介於33-45座可使社會總成本達最小化。 A new intercity bus service, so-called "President Seat Bus" with only 19-21 seats in a regular bus, was introduced to the intercity transportation market since 1998. For maintaining a given service capacity, buses with less number of seats must short bus headway and increase their service frequency. Then, the passenger will benefit from reducing their waiting time due to shorter headway. However, the bus fleet, operation cost, and fare will also increase due to the change of the number of seats of intercity buses. From a minimum social cost point of view, the purpose of this study is to find an optimize solution which is the trade-off between the passenger advantages and operator's cost. Two analytic models are developed to find the optimization number of seats for intercity bus, one basic model consider the operators' and passengers' cost, the other model consider the external (congestion) cost in addition. The basic assumptions of analytic model include unlimited fleet, constant arrival rate within the short period. The results shows that the optimization number of seats increases as the passenger demand, the bus circulation time, the occupancy, and the bus fixed/variable cost increase, and decreases as the value of waiting time, the ratio of waiting time to headway, and load factor decrease. The optimization of the number of seats in Taipei-Taichung line is estimated 33-45 seats under the existing market demand and assumption.