New vibration damping plastic alloys were developed using blends of nylon-6 or polypropylene (PP) with modifiers and damping agent. The damping properties and stiffness were compared with commercially available nylon-6 and PP. As trials, we have attempted to make these plastic alloys construction of engine such as cylinder head cover and timing velt cover. These plastic alloys were molded into engine parts such as the front cover, rocker cover and timing belt cover. The purpose was to reduce the noise levels approximately 4.0 dB lower than that of aluminum covers. The newly developed plasitic alloy engine covers have great potential to reduce engine noise.