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.