The structure of composite insulator consists of following basic parts: 1) CORE is fiberglass reinforced plastics rod which produced from the highest quality materials .It is the component to bear mechanical load and has a minimum tension strength at 1000 Mpa. 2) ENDFITTINGS are made of steel with hot-dip galvanized directly to the rod by a special process. 3) SHEATH & WEATHERSHEDS are produced with high temperature vulcanized silicone rubber compound which exhibit high , long-term hydrophobicity ,high aging resistance , high corona resistance and low permeability to moisture. 4) CORONA RING-is made of aluminum. The corona ring is necessary at 220kV system above .For application at 220kV and 230 kV, one corona ring should be assembled at the line end of the suspension insulator ;For application at 345kV and more , two corona rings should be assembled at both end of the insulators .