As generally known that concrete has low resistance to acidic and corrosive materials which usually exist in acidic and saline environment. And along the increasing of environment conservation, specifically about reusing a waste which is residue of an activity and/or a production process of industries such as metallic, mining, chemical, etc., that pollute the living environment without proper treatment as known as B3 wastes. Among materials belong to B3 waste category are Ground Granulate Blast Furnace Slag (GGBFS) and Fly Ash (FA). The objective of this study is to use GGBFS and FS as an additive and replacement materials of cement and to find the effect of these material to flexural strength of cement. This study uses a beam-shaped specimen with 10 mixed variations consisting of normal concrete (0%), BFG (2%, 2%), BFG (2%, 4%), BFG (2%, 6%), BFG (4%, 2%), BFG (4%, 4%), BFG (4%, 6%), BFG (6%, 2%), BFG (6%, 4%), BFG (6%, 6% ) of cement weight. The results of flexural strength testing show that the mix variation of FA and GGBFS will be increase the average flexural strength of concrete than normal concrete without partial replacement of cement, and the optimum flexural strength had resulted by maximum addition of GGBFS content. Based on that, the material which is more preferable as partial replacement of OPC is GGBFS.