Uniform cooling inside a large bus in hot season is important for comfort of all the passengers. The air, which is cooled by the car cooling system, is distributed into the room space of the bus by a duct system, For some buses with a sub-engine cooling system, however, it is cold at front seats while there is little cooling at rear seats. In order to solve this problem, several factors such as equal distribution of cooling air at each nozzle, duct insulation, exchange rate of room air, heat generation of reading lamps, etc. may be studied. Among these factors, the most important one is equal distribution of the cooling air, since the cooling effect is mainly due to the cooling air stream. In this study, the duct system installed in a specific bus is treated by using a CFD code. The cooling air velocity at the foremost nozzle of the current system is 1.5-2 times that in the middle. This is improved to have almost equal velocity at each nozzle by modifying the duct shape and air flow resistance inside the system. The CFD results for air velocity at each nozzle before and after the improvement are compared with those by vehicle tests to verify the validity of the analysis.