Concrete is the most durable, versatile and reliable construction material after water. Its requirement is very large so the need of ordinary portland cement is also high. Due to such large production of OPC our environment is getting polluted as carbon dioxide is emitted. Geo-polymer is the alternative to control the pollution and its an innovative construction material. It can be prepared by using fly ash, the secondary product of thermal coal and steel slag which is the by product of the production of steel in an electric arc. Another thing required to prepare the geopolymer concrete (GPC) is curing temperature. In this study mechanical properties of GPC is examined with curing temperature 70°C with a replacement of steel slag as coarse aggregate as 0%, 10%, 20%, 30% and 40%. Here the strength of GPC is increasing up to 30% replacement of steel slag as coarse aggregate.