In order to calculate efficiently the characteristics of car body vibration and the acoustic characteristic of the passenger compartment, a structural-acoustic analysis system, ‘CAD-B’, was developed. This system divides the body into three components - front body, main cabin and rear body. The characteristics of front and rear body vibration are expressed in modal parameters. The vibration characteristic throughout the car body is then calculated through the building block approach, while the main cabin remains in finite elements. A good agreement in eigen pairs was seen between this approach and the conventional finite element method.As for the passenger compartment, it is divided into finite elements and its eigen pairs are calculated. Then by linking body vibration with the acoustic characteristic of the passenger compartment, sound pressure in the passenger compartment is calculated.Since in the CAD-B system, the contribution of each panel vibration to sound pressure is calculated and displayed, the parts of the panel to be deadened are derived quickly. This approach has been applied to the problem of road noise reduction, and resultantly, interior noise could be reduced by using only a small quantity of sound deadening material. Some floor panel modifications to reduce noise were also made.