The primary purpose of using a plastic material instead of conventional aluminum cast for intake manifold is to reduce its weight and cost. Moreover, the use of plastic for intake manifold is regarded as a key for further development of so called an “intake modular system”. As a secondary effect, the engine power can be increased with the help of improved interior surface roughness and lowered air temperature. With regard to NVH, however, plastic intake manifold is considered somewhat negative since it is less rigid and less dense than aluminum one.In this paper, the mechanism that plastic intake manifold affects the performance and NVH of in-line 4 cylinder gasoline engine is presented. In connection with engine performance, air flow efficiency of not only intake manifold itself but also other components of intake system and also cylinder head is evaluated. The difference of intake air temperature passing through plastic and aluminum manifold is measured when the engine is operated from cold start and in hot WOT(wide open throttle) condition. As for NVH, natural frequencies and damping ratio of manifolds are measured by impact hammering method and then the measurements of level of noise and vibration on an engine dynamometer and a vehicle are accompanied.The manifold is made by two injection molded pieces and assembled by vibration welding technique.