Disclosed is a heavyweight concrete composition using slag byproducts, including: ground granulated blast furnace slag; slow-cooled electric-arc-furnace oxidizing slag or atomized steel slag used as a fine aggregate; a coarse aggregate; water; and a chemical admixture based on 100 parts by weight high-early-strength cement. Thus, the heavyweight concrete composition contains a large quantity of steel slag and blast-furnace slag that are byproducts in iron-making and steel-making processes, thereby remarkably reducing an amount of emission of carbon dioxide and maximizing the recycling of waste resources for preserving the natural environment. As a result, an eco-friendly heavyweight concrete product can be made.