The presented bolt cap product is developed in South Korea with Twocap's own technology for the purpose to prevent bolt & nut corrosion. The bolt cap is in dual structure of an external cap and internal cap and they prevent the bolt and nut from getting humidified and resulting corrosion. The product also prevents bolt cap from natural damage caused by the sunlight which evaporates oil-based ingredients and makes bolt cap contracted. The product is also simple to use by hitting with a rubber hammer and thus, saves cost. It can be used regardless of inch bolt or mm bolt and guarantees excellent performance with any of them. It is to be mounted on the bolt and nut fixing part and for the purposes of thermal, water, wind, and solar power generation; water supply and sewage plumbing, bridges, road facilities, urban structures, vessels, street lights, traffic lights, park.