As the global environmental protection becomes the world consensus recently, the regulations of the fuel consumption and the exhaust gas have large effects on the performance and the fundamental structure of commercial vehicles. Especially the technology concerning "fluid" and "heat" has a close relationship with those issues. Owing to above circumstances, commercial vehicles such as large trucks and buses are forced to be designed near the limit of allowance. Furthermore, a rapid design is another requirement. However, though significant number of variations, i.e., cab configuration, wheel base, rear body configuration, engine specification, etc., are prepared, it is impossible to improve the performance of all those combinations by experiments which cost a lot. Accordingly, the quantitative prediction using computer will become indispensable at the beginning term of new car development.Then, the authors have completed a new analyzing method to evaluate aerodynamic drag, engine cooling performance and practical problem caused by air flow around the vehicle numerically with the CFD, which will enable to design "well-born" structured vehicles. For putting this method to practical use, it has been essential to develop technologies to calculate the flow and thermal field around the vehicle and in the engine compartment accurately. This paper introduces outline of those methods.Finally, it is confirmed that the procedure suggested in this paper makes it possible to investigate several kinds of performance concurrently, which are the styling design, the layout in the engine compartment and the specification of the engine, without conducting parameter surveys using a prototype vehicle, so that rapid and optimal development will be available.